REGISTRATION NO. 333-
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                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                             ----------------------

                                    FORM S-3
                             REGISTRATION STATEMENT
                                      UNDER
                           THE SECURITIES ACT OF 1933

                             ----------------------

                                TETRA TECH, INC.
             (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)

             DELAWARE                                   95-4148514
   (STATE OR OTHER JURISDICTION OF                    (IRS EMPLOYER
    INCORPORATION OR ORGANIZATION)                  IDENTIFICATION NO.)

                          670 NORTH ROSEMEAD BOULEVARD
                           PASADENA, CALIFORNIA 91107
                                 (626) 351-4664
          (ADDRESS, INCLUDING ZIP CODE, AND TELEPHONE NUMBER, INCLUDING
             AREA CODE, OF REGISTRANTS' PRINCIPAL EXECUTIVE OFFICES)

                             ----------------------

                                  LI-SAN HWANG
                      PRESIDENT AND CHIEF EXECUTIVE OFFICER
                                TETRA TECH, INC.
                          670 NORTH ROSEMEAD BOULEVARD
                           PASADENA, CALIFORNIA 91107
                                 (626) 351-4664
            (NAME, ADDRESS, INCLUDING ZIP CODE, AND TELEPHONE NUMBER,
                   INCLUDING AREA CODE, OF AGENT FOR SERVICE)

                             ----------------------

                                   COPIES TO:
                                 JANIS B. SALIN
                               Riordan & McKinzie
                             300 South Grand Avenue
                                   29th Floor
                          Los Angeles, California 90071

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     APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC: As soon as
PRACTICABLE after this Registration Statement becomes effective.

     If the only securities being registered on this Form are being offered
pursuant to dividend or interest reinvestment plans, please check the following
box. |_|

     If any of the securities being registered on this Form are to be offered on
a delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933, other than securities offered only in connection with dividend or interest
reinvestment plans, check the following box. |X|

     If this Form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, please check the following box
and list the Securities Act registration statement number of the earlier
effective registration statement for the same offering. |_|

     If this Form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. |_|

     If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box. |_|




                                            CALCULATION OF REGISTRATION FEE
===================================================================================================================================

           TITLE OF EACH CLASS OF                   AMOUNT           PROPOSED MAXIMUM      PROPOSED MAXIMUM          AMOUNT OF
              SECURITIES TO BE                       TO BE            OFFERING PRICE           AGGREGATE           REGISTRATION
                 REGISTERED                       REGISTERED            PER UNIT(1)        OFFERING PRICE(1)          FEE(1)
-----------------------------------------------------------------------------------------------------------------------------------
                                                                                                       
Common Stock, $.01 par value...............     127,547 shares            $26.09              $3,327,701               $832
===================================================================================================================================


(1) Estimated solely for the purpose of calculating the registration fee
    pursuant to Rule 457.

    THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR
DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL
FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION
STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(a) OF
THE SECURITIES ACT OF 1933 OR UNTIL THIS REGISTRATION STATEMENT SHALL BECOME
EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SAID SECTION 8(a),
MAY DETERMINE.
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         The information in this prospectus is not complete and may be changed.
These securities may not be sold until the registration statement filed with the
Securities and Exchange Commission is effective. This prospectus is not an offer
to sell securities, and we are not soliciting offers to buy these securities, in
any state where the offer or sale is not permitted.

PROSPECTUS                                                 SUBJECT TO COMPLETION
                                                           DATED MAY 16, 2001




                                TETRA TECH, INC.

                         127,547 SHARES OF COMMON STOCK

                             ----------------------

         The stockholders of Tetra Tech, Inc. listed herein are offering and
selling 127,547 shares of Common Stock of Tetra Tech, Inc. under this
prospectus.

                             ----------------------

           INVESTING IN TETRA TECH, INC. COMMON STOCK INVOLVES RISKS.
                      SEE RISK FACTORS BEGINNING ON PAGE 2.

                             ----------------------

         The selling stockholders obtained their shares of Common Stock on March
30, 2001 in connection with Tetra Tech, Inc.'s acquisition of Williams, Hatfield
& Stoner, Inc. as of March 30, 2001. Some or all of the selling stockholders
expect to sell their shares.

         The selling stockholders may offer their shares of Common Stock through
public or private transactions, on or off the Nasdaq National Market, at
prevailing market prices, or at privately negotiated prices.

                             ----------------------

         Tetra Tech, Inc. Common Stock is traded on the Nasdaq National
Market under the symbol "TTEK." On May 15, 2001 the closing price of the
Common Stock on the Nasdaq National Market was $26.09 per share.

                             ----------------------

THE SECURITIES AND EXCHANGE COMMISSION AND STATE SECURITIES REGULATORS HAVE NOT
APPROVED OR DISAPPROVED THESE SECURITIES, OR DETERMINED IF THIS PROSPECTUS IS
TRUTHFUL OR COMPLETE. ANY REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.

                             ----------------------

                   THE DATE OF THIS PROSPECTUS IS MAY 16, 2001



                                   THE COMPANY

        Tetra Tech, Inc. is a leading provider of specialized management
consulting and technical services in three principal business areas: resource
management, infrastructure and communications. As a specialized management
consultant, we assist our clients in defining problems and developing innovative
and cost-effective solutions. Our management consulting services are
complemented by our technical services. These technical services, which
implement solutions, include research and development, applied science,
engineering and architectural design, construction management, and operations
and maintenance. Our clients include a diverse base of public and private
organizations located in the United States and internationally.

        Since our initial public offering in December 1991, we have increased
the size and scope of our business and have expanded our service offerings
through a series of strategic acquisitions and internal growth. We have more
than 6,500 employees worldwide, of which approximately 6,400 are located in
North America in more than 150 locations. In addition, we have established a
presence in Asia, South America and Europe.

        Our principal executive offices are located at 670 North Rosemead
Boulevard, Pasadena, California 91107, and our telephone number is (626)
351-4664. Our website is located at www.tetratech.com. Information contained in
our website is not a part of this prospectus.

                                 USE OF PROCEEDS

        The selling stockholders are offering all of the shares of Common Stock
covered by this prospectus. We will not receive any proceeds from the sales of
these shares.

                                  RISK FACTORS

        AN INVESTMENT IN THE SHARES OF COMMON STOCK OFFERED BY THIS PROSPECTUS
INVOLVES A HIGH DEGREE OF RISK. YOU SHOULD CAREFULLY REVIEW THE FOLLOWING RISK
FACTORS AS WELL AS THE OTHER INFORMATION SET FORTH IN THIS PROSPECTUS BEFORE
MAKING AN INVESTMENT.

        SOME OF THE INFORMATION IN THIS PROSPECTUS OR INCORPORATED BY REFERENCE
IN THIS PROSPECTUS CONTAINS FORWARD-LOOKING STATEMENTS THAT INVOLVE SUBSTANTIAL
RISKS AND UNCERTAINTIES. YOU CAN IDENTIFY THESE STATEMENTS BY FORWARD-LOOKING
WORDS SUCH AS "MAY," "WILL," "EXPECT," "ANTICIPATE," "BELIEVE," "ESTIMATE" AND
"CONTINUE" OR SIMILAR WORDS. YOU SHOULD READ STATEMENTS THAT CONTAIN THESE WORDS
CAREFULLY BECAUSE THEY: (1) DISCUSS OUR FUTURE EXPECTATIONS; (2) CONTAIN
PROJECTIONS OF OUR FUTURE OPERATING RESULTS OR OF OUR FUTURE FINANCIAL
CONDITION; OR (3) STATE OTHER "FORWARD-LOOKING" INFORMATION. WE BELIEVE IT IS
IMPORTANT TO COMMUNICATE OUR EXPECTATIONS TO OUR INVESTORS. THERE MAY BE EVENTS
IN THE FUTURE, HOWEVER, THAT WE ARE NOT ACCURATELY ABLE TO PREDICT OR OVER WHICH
WE HAVE NO CONTROL. THE RISK FACTORS LISTED IN THIS SECTION, AS WELL AS ANY
CAUTIONARY LANGUAGE IN THIS PROSPECTUS, PROVIDE EXAMPLES OF RISKS, UNCERTAINTIES
AND EVENTS THAT MAY CAUSE OUR ACTUAL RESULTS TO DIFFER MATERIALLY FROM THE
EXPECTATIONS WE DESCRIBE IN OUR FORWARD-LOOKING STATEMENTS. BEFORE YOU INVEST IN
OUR COMMON STOCK, YOU SHOULD BE AWARE THAT THE OCCURRENCE OF ANY OF THE EVENTS
DESCRIBED IN THESE RISK FACTORS AND ELSEWHERE IN THIS PROSPECTUS COULD HAVE A
MATERIAL ADVERSE EFFECT ON OUR BUSINESS, FINANCIAL CONDITION AND OPERATING
RESULTS AND THAT UPON THE OCCURRENCE OF ANY OF THESE EVENTS, THE TRADING PRICE
OF OUR COMMON STOCK COULD DECLINE AND YOU COULD LOSE ALL OR PART OF YOUR
INVESTMENT.

THERE ARE RISKS ASSOCIATED WITH OUR ACQUISITION STRATEGY THAT COULD ADVERSELY
IMPACT OUR BUSINESS AND OPERATING RESULTS

        A significant part of our growth strategy is to acquire other companies
that complement our lines of business or that broaden our geographic presence.
During fiscal 2000, we purchased nine companies in eight separate transactions.
During the six months ended April 1, 2001, we purchased three companies. We
expect to continue to acquire companies as an element of our growth strategy.
Acquisitions involve certain risks that could cause our actual growth or
operating results to differ from our expectations or the expectations of
security analysts. For example:

        o        We may not be able to identify suitable acquisition candidates
                 or to acquire additional companies on favorable terms;


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        o        We compete with others to acquire companies. Competition may
                 increase and may result in decreased availability or increased
                 price for  suitable acquisition candidates;

        o        We may not be able to obtain the necessary financing, on
                 favorable terms or at all, to finance any of our potential
                 acquisitions;

        o        We may ultimately fail to consummate an acquisition even if we
                 announce that we plan to acquire a company;

        o        We may fail to successfully integrate or manage these acquired
                 companies due to differences in business backgrounds or
                 corporate cultures;

        o        These acquired companies may not perform as we expect;

        o        We may find it difficult to provide a consistent quality of
                 service across our geographically diverse operations; and

        o        If we fail to successfully integrate any acquired company, our
                 reputation could be damaged. This could make it more difficult
                 to market our services or to acquire additional companies in
                 the future.

In addition, our acquisition strategy may divert management's attention away
from our primary service offerings, result in the loss of key clients or
personnel and expose us to unanticipated liabilities.

        Finally, acquired companies that derive a significant portion of their
revenues from the Federal government and that do not follow the same cost
accounting policies and billing procedures as we do may be subject to larger
cost disallowances for greater periods than we typically encounter. If we fail
to determine the existence of unallowable costs and establish appropriate
reserves in advance of an acquisition we may be exposed to material
unanticipated liabilities, which could have a material adverse effect on our
business.

OUR QUARTERLY OPERATING RESULTS MAY FLUCTUATE SIGNIFICANTLY, WHICH COULD HAVE A
NEGATIVE EFFECT ON THE PRICE OF OUR COMMON STOCK

        Our quarterly revenues, expenses and operating results may fluctuate
significantly because of a number of factors, including:

        o        The seasonality of the spending cycle of our public sector
                 clients, notably the Federal government;

        o        Employee hiring and utilization rates;

        o        The number and significance of client engagements commenced
                 and completed during a quarter;

        o        Delays incurred in connection with an engagement;

        o        The ability of clients to terminate engagements without
                 penalties;

        o        The size and scope of engagements;

        o        The timing of expenses incurred for corporate initiatives;

        o        The timing and size of the return on investment capital; and

        o        General economic and political conditions.

Variations in any of these factors could cause significant fluctuations in our
operating results from quarter to quarter and could result in net losses.


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THE VALUE OF OUR COMMON STOCK COULD CONTINUE TO BE VOLATILE

        The trading price of our Common Stock has fluctuated widely. In
addition, in recent years the stock market has experienced extreme price and
volume fluctuations. The overall market and the price of our Common Stock may
continue to fluctuate greatly. The trading price of our Common Stock may be
significantly affected by various factors, including:

        o        Quarter to quarter variations in our operating results;

        o        Changes in environmental legislation;

        o        Changes in investors' and analysts' perception of the business
                 risks and conditions of our business;

        o        Broader market fluctuations; and

        o        General economic or political conditions.

IF WE ARE NOT ABLE TO SUCCESSFULLY MANAGE OUR GROWTH STRATEGY, OUR BUSINESS AND
RESULTS OF OPERATIONS MAY BE ADVERSELY AFFECTED

        We are growing rapidly. Our growth presents numerous managerial,
administrative, operational and other challenges. Our ability to manage the
growth of our operations will require us to continue to improve our operational,
financial and human resource management information systems and our other
internal systems and controls. In addition, our growth will increase our need to
attract, develop, motivate and retain both our management and professional
employees. The inability of our management to manage our growth effectively or
the inability of our employees to achieve anticipated performance or utilization
levels, could have a material adverse effect on our business.

THE LOSS OF KEY PERSONNEL OR OUR INABILITY TO ATTRACT AND RETAIN QUALIFIED
PERSONNEL COULD SIGNIFICANTLY DISRUPT OUR BUSINESS

        We depend upon the efforts and skills of our executive officers, senior
managers and consultants. With limited exceptions, we do not have employment
agreements with any of these individuals. The loss of the services of any of
these key personnel could adversely affect our business. Although we have
obtained non-compete agreements from certain principals and stockholders of
companies we have acquired, we generally do not have non-compete or employment
agreements with key employees who were not once equity holders of these
companies. We do not maintain key-man life insurance policies on any of our
executive officers or senior managers.

        Our future growth and success depends on our ability to attract and
retain qualified scientists and engineers. The market for these professionals is
competitive and we may not be able to attract and retain such professionals.

CHANGES IN EXISTING LAWS AND REGULATIONS COULD REDUCE THE DEMAND FOR OUR
SERVICES

        A significant amount of our resource management business is generated
either directly or indirectly as a result of existing Federal and state
governmental laws, regulations and programs. Any changes in these laws or
regulations that reduce funding or affect the sponsorship of these programs
could reduce the demand for our services and could have a material adverse
effect on our business.


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OUR REVENUES FROM AGENCIES OF THE FEDERAL GOVERNMENT ARE CONCENTRATED, AND A
REDUCTION IN SPENDING BY THESE AGENCIES COULD ADVERSELY AFFECT OUR BUSINESS AND
OPERATING RESULTS

        Agencies of the Federal government are among our most significant
clients. During the six months ended April 1, 2001, approximately 24.7% of our
net revenue was derived from Federal agencies, of which 13.0% was derived from
the Department of Defense (DOD), 8.4% from the Environmental Protection Agency
(EPA), 1.3% from the Department of Energy (DOE), and 2.0% from various other
Federal government agencies. Some contracts with Federal government agencies
require annual funding approval and may be terminated at their discretion. A
reduction in spending by Federal government agencies could limit the continued
funding of our existing contracts with them and could limit our ability to
obtain additional contracts. These limitations, if significant, could have a
material adverse effect on our business.

        Additionally, the failure of clients to pay significant amounts due us
for our services could adversely affect our business. For example, we recently
received notification from a Federal government agency that we are entitled to
payments in excess of our billings. However, the agency involved must obtain
specific funding approval for amounts owed to us and there can be no assurance
this funding approval will be obtained.

OUR CONTRACTS WITH GOVERNMENTAL AGENCIES ARE SUBJECT TO AUDIT, WHICH COULD
RESULT IN THE DISALLOWANCE OF CERTAIN COSTS

        Contracts with the Federal government and other governmental agencies
are subject to audit. Most of these audits are conducted by the Defense Contract
Audit Agency (DCAA), which reviews our overhead rates, operating systems and
cost proposals. The DCAA may disallow costs if it determines that we accounted
for these costs incorrectly or in a manner inconsistent with Cost Accounting
Standards. A disallowance of costs by the DCAA, or other governmental auditors,
could have a material adverse effect on our business.

OUR BUSINESS AND OPERATING RESULTS COULD BE ADVERSELY AFFECTED BY LOSSES UNDER
FIXED-PRICE CONTRACTS OR TERMINATION OF CONTRACTS AT THE CLIENT'S DISCRETION

        We contract with Federal and state governments as well as with the
commercial sector. These contracts are often subject to termination at the
discretion of the client with or without cause. Additionally, we enter into
various types of contracts with our clients, including fixed-price and
fixed-unit price contracts. In the six months ended April 1, 2001, approximately
46.3% of our net revenue was derived from fixed-price contracts and fixed-unit
price contracts. Fixed-price contracts protect clients and expose us to a number
of risks. These risks include underestimation of costs, problems with new
technologies, unforeseen costs or difficulties, delays beyond our control and
economic and other changes that may occur during the contract period. Losses
under fixed-price contracts or termination of contracts at the discretion of the
client could have a material adverse effect on our business.

OUR INABILITY TO FIND QUALIFIED SUBCONTRACTORS COULD ADVERSELY AFFECT THE
QUALITY OF OUR SERVICE AND OUR ABILITY TO PERFORM UNDER CERTAIN CONTRACTS

        Under some of our contracts, we depend on the efforts and skills of
subcontractors for the performance of certain tasks. Reliance on subcontractors
varies from project to project. During the six months ended April 1, 2001,
subcontractor costs comprised 25.2% of our gross revenue. The absence of
qualified subcontractors with whom we have a satisfactory relationship could
adversely affect the quality of our service and our ability to perform under
some of our contracts.

OUR INDUSTRY IS HIGHLY COMPETITIVE AND WE MAY BE UNABLE TO COMPETE EFFECTIVELY

        We provide specialized management consulting and technical services to a
broad range of public and private sector clients. The market for our services is
highly competitive and we compete with many other firms. These firms range from
small regional firms to large national firms which have greater financial and
marketing resources than ours.

        We focus primarily on the resource management, infrastructure and
communications business areas. We provide services to our clients which include
Federal, state and local agencies, and organizations in the private sector.


                                       5


        We compete for projects and engagements with a number of competitors
which can vary from 10 to 100 firms. Historically, clients have chosen among
competing firms based on the quality and timeliness of the firm's service. We
believe, however, that price has become an increasingly important factor.

        We believe that our principal competitors include, in alphabetical
order, Black & Veatch LLP; Brown & Caldwell; Castle Tower Corporation; CH2M Hill
Companies Ltd.; EA Engineering, Science & Technology, Inc.; Earth Tech, Inc.; IT
Group Inc.; Mastec, Inc.; Montgomery Watson; o2wireless Solutions, Inc.; Quanta
Services; Roy F. Weston, Inc.; Science Applications International Corporation;
URS Corporation and Wireless Facilities, Inc.

OUR SERVICES EXPOSE US TO SIGNIFICANT RISKS OF LIABILITY AND OUR INSURANCE
POLICIES MAY NOT PROVIDE ADEQUATE COVERAGE

        Our services involve significant risks of professional and other
liabilities which may substantially exceed the fees we derive from our services.
Our business activities could expose us to potential liability under various
environmental laws such as the Comprehensive Environmental Response,
Compensation and Liability Act of 1980 (CERCLA). In addition, we sometimes
contractually assume liability under indemnification agreements. We cannot
predict the magnitude of such potential liabilities.

        We currently maintain comprehensive general liability, umbrella and
professional liability insurance policies. We believe that our insurance
policies are adequate for our business operations. Professional liability
policies are "claims made" policies. Thus, only claims made during the term of
the policy are covered. Should we terminate our professional liability policy
and not obtain retroactive coverage, we would be uninsured for claims made after
termination even if these claims are based on events or acts that occurred
during the term of the policy. Additionally, our insurance policies may not
protect us against potential liability due to various exclusions and retentions.
In addition, if we expand into new markets, we may not be able to obtain
insurance coverage for such activities or, if insurance is obtained, the dollar
amount of any liabilities incurred could exceed our insurance coverage.
Partially or completely uninsured claims, if successful and of significant
magnitude, could have a material adverse affect on our business.

WE MAY BE PRECLUDED FROM PROVIDING CERTAIN SERVICES DUE TO CONFLICT OF INTEREST
ISSUES

        Many of our clients are concerned about potential or actual conflicts of
interest in retaining management consultants. Federal government agencies have
formal policies against continuing or awarding contracts that would create
actual or potential conflicts of interest with other activities of a contractor.
These policies, among other things, may prevent us from bidding for or
performing contracts resulting from or relating to certain work we have
performed for the government. In addition, services performed for a private
client may create a conflict of interest that precludes or limits our ability to
obtain work from other public or private organizations. We have, on occasion,
declined to bid on projects because of these conflicts of interest issues.

OUR INTERNATIONAL OPERATIONS EXPOSE US TO RISKS SUCH AS FOREIGN CURRENCY
FLUCTUATIONS

        During the six months ended April 1, 2001, approximately 3.8% of our net
revenue was derived from the international marketplace. Some contracts with our
international clients are denominated in foreign currencies. As such, these
contracts contain inherent risks including foreign currency exchange risk and
the risk associated with expatriating funds from foreign countries. If our
international revenue increases, our exposure to foreign currency fluctuations
will also increase. We periodically enter into forward exchange contracts to
address certain foreign currency fluctuations.

                              SELLING STOCKHOLDERS

        We acquired Williams, Hatfield & Stoner, Inc., a Florida corporation
(WHS), under a stock purchase agreement dated March 30, 2001 among Tetra Tech
and the WHS shareholders (the "WHS Acquisition"). In connection with the WHS
Acquisition, we issued to the WHS shareholders an aggregate of 127,547 shares of
our Common Stock.

        Under a Registration Rights Agreement dated as of March 30, 2001, we
agreed to register the shares of Common Stock issued to the former WHS
shareholders and to use commercially reasonable efforts to keep the


                                       6


registration statement effective until the date on which all selling
stockholders may sell their shares of Common Stock under Rule 144 promulgated
under the Securities Act of 1933, as amended (the "Securities Act"), without any
volume limitation. Our registration of the shares of Common Stock does not
necessarily mean that the selling stockholders will sell all or any of the
shares.

        The shares listed below represent all of the shares that each selling
stockholder currently owns of our Common Stock. Except as otherwise noted, we
know of no agreements among our stockholders which relate to voting or
investment power over our Common Stock. Except as otherwise noted, the address
of each selling stockholder is c/o Williams, Hatfield & Stoner, Inc., 2101 N.
Andrews Avenue, Suite 300, Ft. Lauderdale, Florida 33311-3940.




                                                                                 SHARES
                                                                               BENEFICIALL           NUMBER OF
                                   NAMES                                       OWNED PRIORY   %(1)    SHARES
                                                                               TO OFFERING            OFFERED
----------------------------------------------------------------------------   ------------   -----   --------
                                                                                             

Anthony A. Nolan............................................................        66,779        *      66,779

James F. Eberhart...........................................................        33,167        *      33,167

Dan B. Glerum...............................................................         6,789        *       6,789

Nancy L. Hurlbert...........................................................        11,575        *      11,575

Lyndon M. Carey.............................................................         5,787        *       5,787

Steven M. Watts.............................................................         3,450        *       3,450


--------------

*       Represents less than 1% of the outstanding shares of Common Stock.

        The selling stockholders listed above are employees of WHS. No selling
stockholder has had any material relationship with us, or any of our
predecessors or affiliates, other than as an employee. Because the selling
stockholders may sell all or part of their shares of Common Stock offered
hereby, no estimate can be given as to the number of shares of Common Stock that
will be held by any selling stockholder upon termination of any offering made
hereby.


                                       7


                              PLAN OF DISTRIBUTION

        We are registering the shares of Common Stock on behalf of the selling
stockholders. As used herein, "selling stockholders" includes donees and
pledgees selling shares received from a named selling stockholder after the date
of this Prospectus. This Prospectus may also be used by transferees of the
selling stockholders or by other persons acquiring shares, including brokers who
borrow the shares to settle short sales of shares of Common Stock. We will bear
all costs, expenses and fees in connection with the registration of the shares
offered hereby. The selling stockholders will bear brokerage commissions and any
similar selling expenses associated with the sale of shares.

        The selling stockholders may offer their shares of Common Stock at
various times in one or more of the following transactions:

        o        on the Nasdaq National Market;

        o        in the over-the-counter market;

        o        in transactions other than on the Nasdaq National Market or in
                 the over-the-counter market;

        o        in connection with short sales of the shares of Common Stock;

        o        by pledge to secure debts and other obligations;

        o        in connection with the writing of non-traded and exchange-
                 traded call options, in hedge transactions and in settlement
                 of other transactions in standardized or over-the-counter
                 options; or

        o        in any combination of any of the above transactions.

        In connection with hedging transactions, broker-dealers or other
financial institutions may engage in short sales of the Common Stock in the
course of hedging the positions they assume with selling stockholders. The
selling stockholders may also enter into options or other transactions with
broker-dealers or other financial institutions, which require the delivery to
such broker-dealer or other financial institution of the shares offered hereby,
which shares may be resold pursuant to this prospectus (as supplemented or
amended to reflect such transaction).

        The selling stockholders may sell their shares at market prices
prevailing at the time of sale, at prices related to such prevailing market
prices, at negotiated prices or at fixed prices. The selling stockholders may
use broker-dealers to sell their shares. If this happens, broker-dealers will
either receive discounts or commissions from purchasers of shares for whom they
acted as agents.

        The selling stockholders have advised us that they have not entered into
any agreements, understandings or arrangements with any underwriters or
broker-dealers regarding the sale of their securities, nor is there an
underwriter or coordinating broker acting in connection with the proposed sale
of shares by the selling stockholders.

        The selling stockholders and any broker-dealers that act in connection
with the sale of shares might be deemed to be "underwriters" within the meaning
of Section 2(11) of the Securities Act, and any commissions received by such
broker-dealers and any profit on the resale of the shares sold by them while
acting as principals might be deemed to be underwriting discounts or commissions
under the Securities Act. We have agreed to indemnify each selling stockholder
against certain liabilities, including liabilities arising under the Securities
Act. The selling stockholders may agree to indemnify any agent, dealer or
broker-dealer that participates in transactions involving sales of the shares
against certain liabilities, including liabilities arising under the Securities
Act.


                                       8


        Because the selling stockholders may be deemed to be "underwriters"
within the meaning of Section 2(11) of the Securities Act, the selling
stockholders will be subject to the prospectus delivery requirements of the
Securities Act. We have informed the selling stockholders that the
anti-manipulative provisions of Regulation M promulgated under the Securities
Exchange Act of 1934, as amended (the "Exchange Act"), may apply to their sales
in the market.

        The selling stockholders also may resell all or a portion of the shares
in open market transactions in reliance upon Rule 144 under the Securities Act,
provided they meet the criteria and conform to the requirements of such Rule.

        Upon being notified by a selling stockholder that any material
arrangement has been entered into with a broker-dealer for the sale of shares
through a block trade, special offering, exchange distribution or secondary
distribution or a purchase by a broker or dealer, we will file a supplement to
this Prospectus, if required, pursuant to Rule 424(b) under the Securities Act,
disclosing (i) the name of each such selling stockholder and of the
participating broker-dealer(s), (ii) the number of shares involved, (iii) the
price at which such shares were sold, (iv) the commissions paid or discounts or
concessions allowed to such broker-dealer(s), where applicable, and (v) other
facts material to the transaction. In addition, upon being notified by a selling
stockholder that a donee or pledgee intends to sell more than 500 shares, we
will file a supplement to this Prospectus.

                       WHERE YOU CAN FIND MORE INFORMATION

        We file annual, quarterly and special reports, proxy statements and
other information with the Securities and Exchange Commission (SEC). You may
read and copy any document we file at the SEC's public reference room at 450
Fifth Street, N.W., Washington, D.C. 20549. Please call the SEC at
1-800-SEC-0330 for further information on the public reference room. Our SEC
filings are also available to the public from the SEC's Website at
"http: //www.sec.gov."

        The SEC allows us to "incorporate by reference" the information we file
with them, which means that we can disclose important information to you by
referring you to those documents. The information incorporated by reference is
considered to be part of this prospectus, and information that we file later
with the SEC will automatically update and supersede this information. We
incorporate by reference the documents listed below and any future filings we
will make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange
Act:

        1.   Annual Report on Form 10-K for the fiscal year ended October 1,
             2000, as filed with the SEC on December 29, 2000;

        2.   Quarterly Report on Form 10-Q for the fiscal quarter ednd April 1,
             2001, as filed with the SEC on May 16, 2001

        3.   Definitive Proxy Statement as filed with the SEC on December 29,
             2000, for the 2001 Annual Meeting of Stockholders; and

        4.   The description of the Common Stock set forth in the Registration
             Statement on Form 8-A dated November 13, 1991, including any
             amendments or reports filed for the purpose of updating such
             description.

        You may request a copy of these filings, at no cost, by writing or
telephoning James M. Jaska as follows:

                                    Tetra Tech, Inc.
                                    Attention:  Investor Relations
                                    670 North Rosemead Boulevard
                                    Pasadena, California 91107
                                    (626) 351-4664


                                       9


        This prospectus is part of a registration statement we filed with the
SEC. You should rely only on the information or representations provided in this
prospectus. We have authorized no one to provide you with different information.
We are not making an offer of these securities in any state where the offer is
not permitted. You should not assume that the information in this prospectus is
accurate as of any date other than the date on the front of the document.

                                  LEGAL MATTERS

        The validity of the Common Stock offered hereby will be passed on for us
by Riordan & McKinzie, a Professional Corporation, Los Angeles, California.
Certain principals of Riordan & McKinzie own shares of Common Stock.

                                     EXPERTS

The financial statements and the related financial statement schedule
incorporated in this prospectus by reference from our Annual Report on Form 10-K
for the year ended October 1, 2000 have been audited by Deloitte & Touche LLP,
independent auditors, as stated in their reports, which are incorporated herein
by reference, and have been so incorporated in reliance upon the reports of such
firm given upon their authority as experts in accounting and auditing.


                                       10


                                     PART II

                     INFORMATION NOT REQUIRED IN PROSPECTUS

ITEM 14. OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION.

     The following is a statement of estimated expenses to be paid by the
Registrant in connection with the issuance and distribution of the securities
being registered.


                                                                                                
            SEC registration fee.................................................................  $     832

            Legal fees...........................................................................      5,000

            Accountants' fees....................................................................      2,000

            Blue Sky qualification fees and expenses.............................................      1,000

            Transfer Agent fees..................................................................      1,000

            Miscellaneous........................................................................      1,000
                                                                                                   ---------

                 Total...........................................................................  $  10,832
                                                                                                   =========


     All of the above amounts, except for the SEC registration fee, have been
estimates.


ITEM 15.  INDEMNIFICATION OF DIRECTORS AND OFFICERS

     Section 145 of the Delaware Corporation Law provides that a Delaware
corporation may indemnify any person against expenses, judgments, fines and
settlements actually and reasonably incurred by any such person in connection
with a threatened, pending or completed action, suit or proceeding in which he
is involved by reason of the fact that he is or was director, officer, employee
or agent of such corporation, provided that (i) he acted in good faith and in a
manner reasonably believed to be in or not opposed to the best interests of the
corporation and (ii) with respect to any criminal action or proceeding, he had
no reasonable cause to believe his conduct was unlawful. If the action or suit
is by or in the name of the corporation, the corporation may indemnify any such
person against expense actually and reasonably incurred by him in connection
with the defense or settlement of such action or suit if he acted in good faith
and in a manner he reasonably believed to be in or not opposed to the best
interests of the corporation, except that no indemnification may be made in
respect to any claim, issue or matter as to which such person shall have been
adjudged to be liable to the corporation for negligence or misconduct in the
performance of his duty to the corporation, unless and only to the extent that
the Delaware Court of Chancery or the court in which the action or suit is
brought determines upon application that, despite the adjudication of liability
but in view of all of the circumstances of the case, such person is fairly and
reasonably entitled to indemnity for such expense as the court deems proper.

     The Company's By-Laws provides for indemnification of persons to the
fullest extent permitted by the Delaware Corporation Law.

     In accordance with the Delaware Corporation Law, the Company's Certificate
of Incorporation, as amended, limits the personal liability of its directors for
violations of their fiduciary duty. The Certificate of Incorporation eliminates
each director's liability to the Company or its stockholders for monetary
damages except (i) for any breach of the director's duty of loyalty to the
Company or its stockholders, (ii) for acts or omissions not in good faith or
which involve intentional misconduct or a knowing violation of law, (iii) under
the section of the Delaware law providing for liability of directors for
unlawful payment of dividends or unlawful stock purchases or redemptions, or
(iv) for any transaction from which a director derived any improper personal
benefit. The effect of this provision is to eliminate the personal liability of
directors for monetary damages for actions involving a breach of their fiduciary
duty of care, including any such actions involving gross negligence. This
provision will not, however, limit in any way the liability of directors for
violations of the Federal securities laws.


                                      II-1


ITEM 16. EXHIBITS.




EXHIBIT
NUMBER    DESCRIPTION
       
   5      Opinion of Riordan & McKinzie, a Professional Corporation.

  23.1    Consent of Deloitte & Touche LLP.

  23.3    Consent of Riordan & McKinzie (included in Exhibit 5).

  24      Powers of Attorney with respect to the Company (included on page II-4).


ITEM 17. UNDERTAKINGS.

         The undersigned registrant hereby undertakes:

         (1) To file, during any period in which offers or sales are being made,
a post-effective amendment to this registration statement:

                  (i) To include any prospectus required by Section 10(a)(3) of
the Securities Act;

                  (ii) To reflect in the prospectus any facts or events arising
after the effective date of the registration statement (or the most recent
post-effective amendment thereof) which, individually or in the aggregate,
represent a fundamental change in the information set forth in the registration
statement;

                  (iii) To include any material information with respect to the
plan of distribution not previously disclosed in the registration statement or
any material change to such information in the registration statement.

PROVIDED, HOWEVER, that paragraphs (1)(i) and (1)(ii) do not apply if the
information required to be included in a post-effective amendment by those
paragraphs is contained in periodic reports filed with or furnished to the
Commission by the registrant pursuant to Section 13 or Section 15(d) of the
Securities Exchange Act of 1934 that are incorporated by reference in the
registration statement.

         (2) That, for the purpose of determining any liability under the
Securities Act, each such post-effective amendment shall be deemed to be a new
registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial BONA
FIDE offering thereof.

         (3) To remove from registration by means of a post-effective amendment
any of the securities being registered which remain unsold at the termination of
the offering.

         (4) That, for purposes of determining any liability under the
Securities Act, each filing of the registrant's annual report pursuant to
Section 13(a) or Section 15(d) of the Securities Exchange Act of 1934 (and,
where applicable, each filing of an employee benefit plan's annual report
pursuant to Section 15(d) of the Securities Exchange Act of 1934) that is
incorporated by reference in the registration statement shall be deemed to be a
new registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial BONA
FIDE offering thereof.

         Insofar as indemnification for liabilities arising under the Securities
Act may be permitted to directors, officers and controlling persons of the
registrant pursuant to the foregoing provisions, or otherwise, the registrant
has been advised that in the opinion of the Securities and Exchange Commission
such indemnification is against public policy as expressed in the Securities Act
and is, therefore, unenforceable. In the event that a claim for indemnification
against such liabilities (other than the payment by the registrant of expenses
incurred or paid by a


                                      II-2


director, officer, or controlling person of the registrant in the successful
defense of any action, suit or proceeding) is asserted by such director, officer
or controlling person in connection with the securities being registered, the
registrant will, unless in the opinion of its counsel the matter has been
settled by controlling precedent, submit to a court of appropriate jurisdiction
the question whether such indemnification by it is against public policy as
expressed in the Securities Act and will be governed by the final adjudication
of such issue.


                                      II-3


                                   SIGNATURES

         Pursuant to the requirements of the Securities Act of 1933, the
Registrant has duly caused this Registration Statement to be signed on its
behalf by the undersigned, thereunto duly authorized in the City of Pasadena,
State of California on the 16th day of May, 2001.

                                  TETRA TECH, INC.


                                       By:  /s/ Li-San Hwang
                                           -------------------------------------
                                                      Li-San Hwang
                                                  Chairman of the Board,
                                               Chief Executive Officer and
                                                       President

     KNOW ALL MEN BY THESE PRESENTS, that each person whose signature appears
below constitutes and appoints Li-San Hwang and James M. Jaska, and each of them
his true and lawful attorneys-in-fact and agents with full power of substitution
and resubstitution, for him and in his name, place and stead, in any and all
capacities, to sign any and all amendments to this Registration Statement,
including any post-effective amendments as well as any related registration
statement (or amendment thereto) filed in reliance upon Rule 462(b) under the
Securities Act of 1933, and to file the same, with all exhibits thereto, and
other documents in connection therewith, with the Securities and Exchange
Commission, granting unto said attorney-in-fact and agents, and each of them,
full power and authority to do and perform each and every act and thing
requisite or necessary to be done in and about the premises, as fully to all
intents and purposes as he might or could do in person, hereby ratifying and
confirming all that said attorneys-in-fact and agents or any of them, or their
or his substitute or substitutes, may lawfully do or cause to be done by virtue
hereof.

     Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the dates indicated.




                  SIGNATURE                                     TITLE                              DATE
                                                                                         
/s/ Li-San Hwang                                                                               May 16, 2001
--------------------------------------------       Chairman of the Board, Chief
                Li-San Hwang                       Executive Officer and President
                                                   (Principal Executive Officer)


/s/ James M. Jaska                                                                             May 15, 2001
---------------------------------------------      Executive Vice President, Chief
               James M. Jaska                      Financial Officer and Treasurer
                                                   (Principal Financial Officer
                                                   and Principal Accounting
                                                   Officer)

/s/ J. Christopher Lewis
---------------------------------------------      Director                                    May 14, 2001
            J. Christopher Lewis


/s/ Patrick C. Haden
---------------------------------------------      Director                                    May 14, 2001
              Patrick C. Haden


/s/ James J. Shelton
---------------------------------------------      Director                                    May 15, 2001
              James J. Shelton


/s/ Daniel A. Whalen
---------------------------------------------      Director                                    May 14, 2001
              Daniel A. Whalen



                                      II-4