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                                EIGHTH AMENDMENT

         THIS EIGHTH AMENDMENT (this "Eighth Amendment") dated as of January 31,
2000 is to the Credit Agreement (the "Credit Agreement") dated as of September
15, 1995 between TETRA TECH, INC. (the "Company") and BANK OF AMERICA, N.A.
(formerly known as Bank of America National Trust and Savings Association) (the
"Bank"). Unless otherwise defined herein, terms defined in the Credit Agreement
are used herein as defined therein.

         WHEREAS, the parties hereto have entered into the Credit Agreement
which provides for the Bank to make Loans to, and to issue Letters of Credit for
the account of, the Company from time to time; and

         WHEREAS, the parties hereto desire to amend the Credit Agreement as
set forth below;

         NOW, THEREFORE, in consideration of the premises and for other good and
valuable consideration (the receipt and sufficiency of which are hereby
acknowledged), the parties hereto agree as follows:

         SECTION 1  AMENDMENTS.  Effective on (and subject to the
occurrence of) the Eighth Amendment Effective Date (as defined below), the
Credit Agreement shall be amended as follows:

         Section 1.1 SECTION 6.1.1.  Section 6.1.1 of the Credit
Agreement is amended in its entirety to read as follows:

                  "6.1.1 SCHEDULED REDUCTIONS OF COMMITMENT.  The amount of
         the Commitment shall be permanently reduced to $60,000,000 on March
         31, 2000."

         Section 1.2 SECTION 5.4. Section 5 of the Credit Agreement is amended
by adding the following in appropriate numerical sequence:

                  "5.4 UTILIZATION FEE. The Company agrees to pay the Bank a
         utilization fee, for each day on or after February 1, 2000 on which the
         sum of (i) the aggregate outstanding principal amount of all Loans PLUS
         (ii) the aggregate Stated Amount of all Letters of Credit exceeds
         $60,000,000, in an amount equal to 0.5% per annum of such excess. The
         utilization fee (a) shall be payable on the last Business Day of each
         month and on the Revolving Termination Date; and (b) shall be computed
         on the basis of a year of 360 days."

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         SECTION 2 REPRESENTATIONS AND WARRANTIES. The Company represents and
warrants to the Bank that (a) each warranty set forth in Section 9 of the
Credit Agreement is true and correct as if made on the date hereof, (b) the
execution and delivery by the Company of this Eighth Amendment and the
performance by the Company of its obligations under the Credit Agreement as
amended hereby (as so amended, the "Amended Credit Agreement") (i) are within
the corporate powers of the Company and each Subsidiary, (ii) have been duly
authorized by all necessary corporate action, (iii) have received all
necessary governmental approval and (iv) do not and will not contravene or
conflict with any provision of law or of the charter or by-laws of the
Company or any Subsidiary or of any indenture, loan agreement or other
material contract, order or decree which is binding upon the Company or any
Subsidiary, and (c) this Eighth Amendment and the Amended Credit Agreement
are the legal, valid and binding obligations of the Company and each
Subsidiary which is party hereto, enforceable against the Company and each
Subsidiary in accordance with their terms, except as enforceability may be
limited by bankruptcy, insolvency or other similar laws of general
application affecting the enforcement of creditor's rights or by general
principles of equity limiting the availability of equitable remedies.

         SECTION 3 EFFECTIVENESS. The amendments set forth in SECTION
1 shall become effective, as of the day and year first above written, on such
date (the "Eighth Amendment Effective Date") that the Bank shall have
received counterparts of this Eighth Amendment executed by the parties hereto.

         SECTION 4 MISCELLANEOUS.

         Section 4.1  CONTINUING EFFECTIVENESS, ETC.  As herein amended,
the Credit Agreement shall remain in full force and effect and is hereby
ratified and confirmed in all respects.

         Section 4.2 COUNTERPARTS. This Eighth Amendment may be executed
in any number of counterparts and by the different parties on separate
counterparts, and each such counterpart shall be deemed to be an original but
all such counterparts shall together constitute one and the same Eighth
Amendment.

         Section 4.3 GOVERNING LAW. This Eighth Amendment shall be a
contract made under and governed by the laws of the State of Illinois
applicable to contracts made and to be performed entirely within such State.

         Section 4.4 SUCCESSORS AND ASSIGNS. This Eighth Amendment shall
be binding upon the Company and the Bank and their respective successors and
assigns, and shall inure to the benefit of the Company and the Bank and the
successors and assigns of the Bank.

         Section 4.5 EXPENSES. The Company agrees to pay the reasonable costs
and expenses of the Agent (including Attorney Costs) in connection with the
preparation, execution and delivery of this Eighth Amendment.

                                      -2-
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    DELIVERED AT CHICAGO, ILLINOIS, AS OF THE DAY AND YEAR FIRST ABOVE WRITTEN.

                                TETRA TECH, INC.


                                BY  /s/ James M. Jaska
                                  --------------------------------------
                                TITLE  Chief Financial Officer
                                     -----------------------------------


                                BANK OF AMERICA, N.A. (FORMERLY BANK OF
                                AMERICA NATIONAL TRUST AND SAVINGS ASSOCIATION)


                                BY  /s/ Jennifer L. Gerdes
                                  --------------------------------------
                                TITLE   Vice President
                                     -----------------------------------




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Each of the undersigned hereby acknowledges and agrees to the foregoing Eighth
Amendment and the Amended Credit Agreement and hereby confirms the continuing
effectiveness of the Guaranty and the Security Agreement with respect to the
Amended Credit Agreement.

                                   HSI GEOTRANS, INC.

                                   BY:  /s/ James M. Jaska
                                   Title:  Asst. Treasurer


                                   SIMONS, LI & ASSOCIATES, INC.

                                   By:  /s/ James M. Jaska
                                   Title:  Asst. Treasurer


                                   TETRA TECH EM, INC.

                                   By:  /s/ James M. Jaska
                                   Title:  Treasurer


                                   WHALEN & COMPANY, INC.

                                   By:  /s/ James M. Jaska
                                   Title:  Chief Financial Officer


                                   TETRA TECH NUS, INC.

                                   By:  /s/ James M. Jaska
                                   Title:  Treasurer


                                   MFG, INC.

                                   By:  /s/ James M. Jaska
                                   Title:  Treasurer




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                                   COLLINS-PINA CONSULTING ENGINEERS, INC.

                                   By:  /s/ James M. Jaska
                                   Title:  Treasurer


                                   DEA CONSTRUCTION COMPANY, INC.

                                   By:  /s/ James M. Jaska
                                   Title:  Treasurer


                                   BAHA COMMUNICATIONS, INC.

                                   By:  /s/ James M. Jaska
                                   Title:  Treasurer


                                   UTILITIES & C.C., INC.

                                   By:  /s/ James M. Jaska
                                   Title:  Treasurer


                                   ASL CONSULTING ENGINEERS, INC.

                                   By:  /s/ James M. Jaska
                                   Title:  Treasurer


                                   COSENTINI ASSOCIATES, INC.

                                   By:  /s/ James M. Jaska
                                   Title:  Treasurer