TETRA TECH INC
POS AM, 1999-02-26
ENGINEERING SERVICES
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<PAGE>

                                       
  AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON FEBRUARY 26, 1999
                                                    REGISTRATION NO. 333-64165
- ------------------------------------------------------------------------------
- ------------------------------------------------------------------------------
                      SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C. 20549
                            ----------------------
                                       
                        POST-EFFECTIVE AMENDMENT NO. 1
                                      TO
                                   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 9110
                                (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
                            ----------------------

     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.  / /

     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.
- ------------------------------------------------------------------------------
- ------------------------------------------------------------------------------

<PAGE>

     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 FEBRUARY 26, 1999
                                          
                                          
                               TETRA TECH, INC.
                                          

                        248,605 SHARES OF COMMON STOCK

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

     The stockholders of Tetra Tech, Inc. listed herein are offering and selling
248,605 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 received their shares of Common Stock on July 
9, 1998 in connection with Tetra Tech, Inc.'s acquisition of McNamee, Porter 
& Seeley, Inc.  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 "WATR."  On February ___, 1999, the closing price of the 
Common Stock on the Nasdaq National Market was $__________ 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 FEBRUARY ___, 1999

<PAGE>

                                  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 3,600 employees worldwide, 3,500 of whom are located in North America in 
more than 100 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.

RISKS ASSOCIATED WITH OUR ACQUISITION STRATEGY

     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 1998, we purchased ten companies in five separate transactions. 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:

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


                                      2

<PAGE>

     -    We compete with others to acquire companies.  We believe that this
          competition will increase and may result in decreased availability or
          increased price for suitable acquisition candidates; 

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

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

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

     -    These acquired companies may not perform as we expect; 

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

     -    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 are.  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. 

FLUCTUATIONS IN OUR QUARTERLY OPERATING RESULTS

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

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

     -    Employee hiring and utilization rates; 

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

     -    Delays incurred in connection with an engagement; 

     -    The ability of our clients to terminate engagements without 
          penalties; 

     -    The size and scope of engagements; 

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

     -    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. 

POTENTIAL VOLATILITY OF OUR STOCK PRICE


                                     3

<PAGE>

     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:

     -    Quarter to quarter variations in our operating results; 

     -    Changes in environmental legislation; 

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

     -    Broader market fluctuations; and 

     -    General economic or political conditions. 

MANAGEMENT OF GROWTH

     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. 

RELIANCE ON KEY PERSONNEL AND QUALIFIED PROFESSIONALS

     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 the principal stockholders of each of 
the companies we have acquired, we generally do not have non-compete or 
employment agreements with key employees who were not 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. 

DEPENDENCE UPON EXISTING LAWS AND REGULATIONS

     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. 

CONCENTRATION OF REVENUES

     Agencies of the Federal government are among our most significant 
clients. During fiscal 1998 and the three months ended January 3, 1999, 
approximately 46.8% and 41.5%, respectively, of our net revenue was derived 
from three federal agencies as follows:  26.2% and 22.8%, respectively, of 
our net revenue was derived from the Department of Defense (DOD), 17.1% and 
14.4%, respectively, from the Environmental Protection Agency (EPA), and 3.5% 
and 3.0%, respectively, from the Department of Energy (DOE).  Some of our 
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. 


                                      4

<PAGE>

     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. 

RISKS ASSOCIATED WITH GOVERNMENTAL AUDITS

     Our 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. 

     In September 1995, we acquired PRC Environmental Management, Inc. (EMI). 
EMI also contracts with Federal government agencies and such contracts are 
also subject to the same governmental audits.  The DCAA has completed audits 
of EMI's contracts for the fiscal years 1987 through 1995.  As a result of 
these audits and our negotiations with the DCAA, the DCAA disallowed 
approximately $2.9 million in costs. 

FIXED PRICE CONTRACTS

     We enter into various contracts with our clients, including fixed-price 
contracts. In fiscal 1998 and the three months ended January 3, 1999, 
approximately 26.1% and 30.4%, respectively, of our net revenue was derived 
from fixed-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. If we incur losses under fixed-price contracts it could have 
a material adverse effect on our business. 

DEPENDENCE ON SUBCONTRACTORS

     Under some of our contracts, we depend on the efforts and skills of 
subcontractors for the performance of certain tasks.  Our reliance on 
subcontractors varies from project to project.  In fiscal 1998 and the three 
months ended January 3, 1999, subcontractor costs comprised 22.3% and 21.7%, 
respectively, 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. 

SIGNIFICANT COMPETITION

     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 we do. 

     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. 

     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; Camp, 
Dresser & McKee; CH2M Hill Companies Ltd.; Dames & Moore Group; EA 
Engineering, Science & Technology, Inc.; Earth Tech, Inc.; ICF Kaiser 
International, Inc.; IT Group Inc.; Mastec, Inc.; Montgomery Watson; OSP 
Consultants, Inc.; Roy F. Weston, Inc.; and URS Greiner Corporation. 


                                      5

<PAGE>
POTENTIAL LIABILITY AND INSURANCE

     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.  These policies are 
"claims made" policies. Thus, only claims made during the term of the policy 
are covered.  If we terminate our policies and do 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.  Our insurance may not protect us against liability because our 
policies typically have 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.  A partially or 
completely uninsured claim, if successful and of significant magnitude, could 
have a material adverse affect on our business. 

CONFLICTS OF INTEREST

     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. 

RISKS ASSOCIATED WITH INTERNATIONAL OPERATIONS

     In fiscal 1998 and the three months ended January 3, 1999, approximately 
3.2% and 5.0%, respectively, 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 have entered into forward exchange contracts to address certain foreign 
currency fluctuations. 

YEAR 2000

     We are working to resolve the potential impact of the year 2000 on our 
business operations and the ability of our computerized information systems 
to accurately process information that may be date-sensitive.  Any of our 
programs that recognize a date using "00" as the year 1900 rather than the 
year 2000 could result in errors or system failures. 

     We utilize a number of computer programs across our entire operation.  
The primary information technology systems we utilize are the accounting and 
financial and human resource information management systems.  We began our 
risk assessment in 1995.  Since that time we have procured and implemented 
certain accounting and financial reporting systems as well as contract 
administration and billing systems that have been certified as year 2000 
compliant by our vendors.  Currently, approximately 72% of our gross revenue 
is recognized on these year 2000 compliant systems.  We believe that our 
financial and accounting and human resource management information systems 
will be year 2000 compliant in a timely manner and will not be materially 
impacted by the year 2000. We may fail, however, in updating our various 
systems to be year 2000 compliant in a timely manner. 

     We have extensive business with the Federal government.  Should the 
Federal government, especially the DOD, experience significant business 
interruptions relating to non-year 2000 compliance, our business could be 

                                       6

<PAGE>

materially impacted. To the extent that other third parties which we rely 
upon, such as banking institutions, clients and vendors, are unable to 
address their year 2000 issues in a timely manner, our business could be 
materially impacted.  We believe that the worst case scenario relating to the 
year 2000 would be an extensive period of time in which the Federal 
government and other third parties could not process payments promptly, in 
addition to our financial institutions not being able to supply us with our 
working capital needs.

     Additional risks associated with non-year 2000 compliance include:

     -    Our inability to invoice and process payments; 

     -    Our inability to produce accurate and timely financials; 

     -    The impact on our profitability; and 

     -    Our potential liability to third parties for not meeting contracted
          deliverables. 

IMPACT OF ANTI-TAKEOVER PROVISIONS ON OUR STOCK PRICE

     Our certificate of incorporation and by-laws and the Delaware General 
Corporation Law include provisions that may be deemed to have anti-takeover 
effects.  These anti-takeover effects could delay or prevent a takeover 
attempt that you or our other stockholders might consider in your or their 
best interests. 

     In addition, our board of directors is authorized to issue, without 
obtaining stockholder approval, up to 2,000,000 shares of preferred stock and 
to determine the price, rights, preferences and privileges of such shares 
without any further stockholder action.  The existence of this "blank-check" 
preferred stock could make more difficult or discourage an attempt to obtain 
control of us by means of a tender offer, merger, proxy contest or otherwise. 

     In the future, we may adopt other measures that may have the effect of 
delaying, deferring or preventing an unsolicited takeover, even if such a 
change in control were at a premium price or favored by a majority of 
unaffiliated stockholders. Certain of these measures may be adopted without 
any further vote or action by the stockholders.


                                     7

<PAGE>
                       PRINCIPAL AND SELLING STOCKHOLDERS

     On July 9, 1998, we completed the acquisition of McNamee, Porter & 
Seeley, Inc., a Michigan corporation ("MPS"), pursuant to the terms of a 
Stock Purchase Agreement dated June 30, 1998 among Tetra Tech and the 
shareholders of  MPS (the "MPS Acquisition").  In connection with the MPS 
Acquisition, we (i) issued to the shareholders of MPS an aggregate of 274,886 
shares of Common Stock and (ii) paid to the shareholders of MPS an aggregate 
of $9,601,972.39 in cash.

     Under a Registration Rights Agreement dated as of July 9, 1998, we 
agreed to register the shares of Common Stock issued to the selling 
stockholders and to use commercially reasonable efforts to keep the 
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. The following table sets 
forth certain information regarding the beneficial ownership of our Common 
Stock at February 17, 1999 and after giving effect to the sale by the selling 
stockholders of 248,605 shares of Common Stock in this offering by: (1) each 
director and executive officer; (2) each person we know to beneficially own 
more than 5% of the outstanding shares of our Common Stock; (3) all officers 
and directors as a group; and (4) the selling stockholders.   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, each person named below has an address in care of our principal 
executive offices.
<TABLE>
<CAPTION>
                                                    Shares                                   Shares
                                                 Beneficially            Number of        Beneficially
                                                 Owned Prior              Shares          Owned After
                   Names                         to Offering    %(1)      Offered           Offering       %(1)
- -----------------------------------------        ------------   ----     ---------        ------------     ----
<S>                                              <C>            <C>      <C>              <C>              <C>
Li-San Hwang(2) . . . . . . . . . . . . .         1,629,961     5.5%         --             1,629,961      5.5%

Daniel A. Whalen(3) . . . . . . . . . . .         1,538,649     5.2          --             1,538,649      5.2
     Whalen & Company, Inc.
     3675 Mt. Diablo Boulevard
     Suite 360
     Lafayette, California 94549

Pilgrim Baxter & Associates, Ltd.(4)  . .         2,727,545     9.2          --             2,727,545      9.2
     825 Duportail Road
     Wayne, Pennsylvania 19087

T. Rowe Price Associates, Inc.(5) . . . .         1,777,555     6.0          --             1,777,555      6.0
     100 E. Pratt Street
     Baltimore, Maryland  21202

J. Christopher Lewis(6) . . . . . . . . .            76,472       *          --                76,472        *

Patrick C. Haden(7) . . . . . . . . . . .            28,145       *          --                28,145        *

James J. Shelton(8) . . . . . . . . . . .            20,869       *          --                20,869        *

Thomas D. Brisbin(9)  . . . . . . . . . .            47,603       *          --                47,603        *

Charles R. Faust(10)  . . . . . . . . . .            48,908       *          --                48,908        *

James M. Jaska(11)  . . . . . . . . . . .            58,209       *          --                58,209        *

William R. Brownlie(12) . . . . . . . . .           147,068       *          --               147,068        *

Steven A. Gherini(13) . . . . . . . . . .           170,262       *          --               170,262        *

Arkan Say(14) . . . . . . . . . . . . . .            39,157       *          --                39,157        *

Richard A. Lemmon(15) . . . . . . . . . .            37,841       *          --                37,841        *
</TABLE>

                                      8
<PAGE>

<TABLE>
<CAPTION>
                                                    Shares                                   Shares
                                                 Beneficially            Number of        Beneficially
                                                 Owned Prior              Shares          Owned After
                   Names                         to Offering    %(1)      Offered           Offering       %(1)
- -----------------------------------------        ------------   ----     ---------        ------------     ----
<S>                                              <C>            <C>      <C>              <C>              <C>
All directors and executive officers 
as a group (12 persons)(16) . . . . . . .         3,843,144     12.9%        --             3,843,144      12.9%

SELLING STOCKHOLDERS 

Khalil Z. Atasi . . . . . . . . . . . . .            14,206       *        14,206                --          *

Dennis J. Benoit  . . . . . . . . . . . .            14,206       *        14,206                --          *

Glenn S. Burkhardt, as Trustee of the
Glenn S. Burkhardt Trust dated
September 12, 1996  . . . . . . . . . . .            26,281       *        26,281                --          *

Thomas M. Doran, as Trustee of the
Thomas M. Doran Trust dated
June 26, 1996   . . . . . . . . . . . . .            26,281       *        26,281                --          *

Charles D. Fifield  . . . . . . . . . . .            22,020       *        22,020                --          *

Richard W. Force, as Trustee of the
Richard W. Force Trust dated
June 9, 1992  . . . . . . . . . . . . . .            26,281       *        26,281                --          *

S. Joh Kang, as Trustee of the Shin Joh
Kang Trust dated January 17, 1992 . . . .            26,281       *        26,281                --          *

Kenneth E. Kingsley . . . . . . . . . . .            14,206       *        14,206                --          *

Donald E. Lund  . . . . . . . . . . . . .            26,281                26,281                --          *

John P. Oyer, as Trustee of the John P.
Oyer Trust dated January 21, 1997 . . . .            26,281       *        26,281                --          *

Philip C. Youngs  . . . . . . . . . . . .            26,281       *        26,281                --          *
</TABLE>
______________

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

(1)  Applicable percentages of ownership are based on 29,749,661 shares of
     Common Stock outstanding on February 17, 1999, adjusted as required by the
     rules promulgated by the Securities and Exchange Commission (SEC).  This
     table is based upon information supplied by officers, directors and
     principal stockholders and Schedules 13D and 13G (if any) filed with the
     SEC.  Unless otherwise indicated, and subject to community property laws
     where applicable, we believe that each of the stockholders named in this
     table has sole voting and investment power with respect to the shares
     indicated as beneficially owned. Any security that any person named above
     has the right to acquire within 60 days is deemed to be outstanding for
     purposes of calculating the percentage ownership of such person, but is not
     deemed to be outstanding for purposes of calculating the ownership
     percentage of any other person. 

(2)  Includes 26,563 shares issuable with respect to stock options exercisable
     within 60 days after February 17, 1999. 

(3)  Includes 3,906 shares issuable with respect to stock options exercisable
     within 60 days after February 17, 1999.  Also includes (a) 1,181,399 shares
     of Common Stock held by Daniel A. Whalen and Katharine C. Whalen, as
     Trustees for the Whalen Family Trust U/A/D 4/30/92, (b) 31,000 shares of
     Common Stock held by Brown Investment Advisory & Trust Company, as Trustee
     for the Whalen 1997 Charitable 


                                       9

<PAGE>

     Remainder Unitrust, (c) 199,498 shares of Common Stock held by DKW/CRT 
     Investments, and (d) 122,846 shares of Common Stock held by D-K-W Ventures 
     LP.

(4)  All information regarding share ownership is taken from and furnished in
     reliance upon the Schedule 13G (Amendment No. 9), dated as of January 19,
     1999, filed by Pilgrim Baxter & Associates, Ltd.

(5)  All information regarding share ownership is taken from and furnished in
     reliance upon the Schedule 13G, dated as of February 12, 1999, filed by T.
     Rowe Price Associates, Inc.

(6)  Includes 19,070 shares issuable with respect to stock options exercisable
     within 60 days after February 17, 1999. 

(7)  Excludes an aggregate of 2,683 shares of Common Stock owned by Mr. Haden's
     wife as to which Mr. Haden disclaims beneficial ownership. Includes 19,070
     shares issuable with respect to stock options exercisable within 60 days
     after February 17, 1999. 

(8)  Includes 5,613 shares held by James J. Shelton, Sarah Belle Shelton and
     James J. Shelton, Jr., Trustees of the James J. Shelton and Sarah Belle
     Shelton Family Trust dated August 19, 1987, and 15,256 shares issuable with
     respect to stock options exercisable within 60 days after February 17,
     1999. 

(9)  Includes 46,835 shares issuable with respect to stock options exercisable
     within 60 days after February 17, 1999. 

(10) Includes 21,230 shares issuable with respect to stock options exercisable
     within 60 days after February 17, 1999.  Additionally, Charles R. Faust's
     minor children own an aggregate of 2,882 shares of Common Stock as to which
     Charles R. Faust disclaims beneficial ownership. 

(11) Includes 57,558 shares issuable with respect to stock options exercisable
     within 60 days after February 17, 1999. 

(12) Includes 41,484 shares issuable with respect to stock options exercisable
     within 60 days after February 17, 1999.  Also includes an aggregate of 400
     shares of Common Stock owned by Dr. Brownlie's minor children. 

(13) Includes 27,090 shares issuable with respect to stock options exercisable
     within 60 days after February 17, 1999. 

(14) Includes 14,157 shares issuable with respect to stock options exercisable
     within 60 days after February 17, 1999. 

(15) Includes 11,945 shares issuable with respect to stock options exercisable
     within 60 days after February 17, 1999. 

(16) Includes 304,164 shares issuable with respect to stock options exercisable
     within 60 days after February 17, 1999.

     All selling stockholders are employees of MPS or trusts of which such
employees are the trustees, and no selling stockholder has had any material
relationship with us, or any of our predecessors or affiliates.  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.

                              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 shareholder 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.

                                      10

<PAGE>

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

     -    on the Nasdaq National Market;

     -    in the over-the-counter market;

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

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

     -    by pledge to secure debts and other obligations;

     -    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

     -    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 shareholders 
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.

     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.


                                      11

<PAGE>

     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 4, 1998
          filed on December 31, 1998; 

     2.   Annual Report on Form 10-K/A (Amendment No. 1) for the fiscal year
          ended October 4, 1998 filed on February 2, 1999;

     3.   Quarterly Report on Form 10-Q for the fiscal quarter ended January 3,
          1999 filed on February 16, 1999.

     4.   Current Report on Form 8-K for the event of September 22, 1998, as
          filed with the SEC on October 7, 1998; 

     5.   Current Report on Form 8-K/A for event of September 22, 1998, as filed
          with the SEC on December 1, 1998; 

     6.   Definitive Proxy Statement, filed on December 31, 1998, for the 1999
          Annual Meeting of Stockholders;

     7.   Definitive Proxy Statement (Amendment No. 1), filed on January 12,
          1999, for the 1999 Annual Meeting of Stockholders; and

     8.   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. 

                                       12

<PAGE>

     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, CA 91107
                         (626) 351-4664

     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, in the aggregate, approximately
162,140 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 4, 1998 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.


                                      13

<PAGE>

                                    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. 

<TABLE>
     <S>                                                               <C>
     SEC registration fee . . . . . . . . . . . . . . . . .            $ 1,523
     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  . . . . . . . . . . . . . . . . . . .            $11,523
                                                                       -------
                                                                       -------
</TABLE>

     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 


                                     II-1

<PAGE>

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.

ITEM 16. EXHIBITS.
<TABLE>
<CAPTION>
EXHIBIT
NUMBER         DESCRIPTION
<S>            <C>
  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).*
</TABLE>
______________
*  Previously filed.

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.


                                     II-2

<PAGE>

     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 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

<PAGE>
                                  SIGNATURES

     Pursuant to the requirements of the Securities Act of 1933, the 
Registrant certifies that it has reasonable grounds to believe that it meets 
all of the requirements for filing on Form S-3 and has duly caused this 
Post-Effective Amendment to the Registration Statement to be signed on its 
behalf by the undersigned, thereunto duly authorized, in the City of 
Pasadena, State of California on the 26th of February, 1999.

                                        TETRA TECH, INC. 


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

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

<TABLE>
<CAPTION>
           SIGNATURE                    TITLE                  DATE
<S>                             <C>                      <C>
/s/ Li-San Hwang                Chairman of the          February 26, 1999
- ----------------------------    Board, Chief
         Li-San Hwang           Executive Officer and
                                President (Principal
                                Executive Officer)

/s/ James M. Jaska              Vice President, Chief    February 26, 1999
- ----------------------------    Financial Officer and
        James M. Jaska          Treasurer (Principal
                                Financial Officer and
                                Principal Accounting
                                Officer)

   *                            Director                 February 26, 1999
- -----------------------------
     J. Christopher Lewis

   *                            Director                 February 26, 1999
- -----------------------------
       Patrick C. Haden

   *                            Director                 February 26, 1999
- -----------------------------
      Joseph J. Shelton

                                Director
- -----------------------------
       Daniel A. Whalen

*By: /s/ Li-San Hwang
     ------------------------
     Li-San Hwang
     Attorney-in-Fact
</TABLE>

<PAGE>

                                                                 EXHIBIT 23.1


INDEPENDENT AUDITORS' CONSENT

We consent to the incorporation by reference in this Post-Effective Amendment 
No. 1 to Registration Statement on Form S-3 (333-64165) of Tetra Tech, Inc. 
on Form S-3 of our reports dated November 13, 1998, appearing in, and 
incorporated by reference in, the Annual Report on Form 10-K of Tetra Tech, 
Inc. for the year ended October 4, 1998, and to the reference to us under the 
heading "Experts" in the Prospectus, which is part of this Registration 
Statement.

     /s/ Deloitte & Touche LLP

DELOITTE & TOUCHE LLP
Los Angeles, California
February 26, 1999




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