Document:

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                                                                   EXHIBIT 10.23

                          REGISTRATION RIGHTS AGREEMENT

                  This Registration Rights Agreement (the "Agreement") is
entered into as of May 3, 2000 by and among Tetra Tech, Inc., a Delaware
corporation ("Tetra Tech"), and the parties listed on SCHEDULE A attached hereto
(each, a "Holder" and collectively, the "Holders").

                                 R E C I T A L S

                  A. Tetra Tech, Tetra Tech Canada Ltd., an Ontario corporation
("TTC"), and the Holders are parties to a Stock Purchase Agreement dated as of
the date hereof (the "Stock Purchase Agreement"); and

                  B. Pursuant to the Stock Purchase Agreement, the Holders will
receive shares of TTC which are exchangeable for shares of the common stock,
$.01 par value, of Tetra Tech ("Tetra Tech Common Stock"); and

                  C. This Agreement is the Registration Rights Agreement
referred to in SECTION 6.2 of the Stock Purchase Agreement and, pursuant
thereto, must be entered into by the parties in connection with the transactions
contemplated by the Stock Purchase Agreement.

                                A G R E E M E N T

                  NOW, THEREFORE, in consideration of the mutual covenants and
agreements contained herein, and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the parties hereto
agree as follows:

                  1. CERTAIN DEFINITIONS. As used in this Agreement, the
following terms shall have the following respective meanings:

                     "EXCHANGE ACT" shall mean the Securities Exchange Act
of 1934, as amended from time to time.

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                           "FORM S-3" shall mean such form under the Securities
Act as in effect on the date hereof or any successor registration form under the
Securities Act subsequently adopted by the SEC which permits inclusion or
incorporation of substantial information by reference to other documents filed
by Tetra Tech with the SEC.

                           "PROSPECTUS" shall mean the prospectus included in
any Registration Statement, as amended or supplemented by any prospectus
supplement, with respect to the terms of the offering of any portion of the
Registrable Securities covered by the Registration Statement and by all other
amendments and supplements to the prospectus, including post-effective
amendments and all material incorporated by reference in such Prospectus.

                           "REGISTER", "REGISTERED" and "REGISTRATION" shall
mean and refer to a registration effected by preparing and filing a Registration
Statement and taking all other actions that are necessary or appropriate in
connection therewith, and the declaration or ordering of effectiveness of such
Registration Statement by the SEC.

                           "REGISTRATION EXPENSES" shall have the meaning set
forth in SECTION 6.

                           "REGISTRABLE SECURITIES" shall mean the shares of
Tetra Tech Common Stock (i) issued upon exchange for the shares of TTC held by
the Holders pursuant to the Stock Purchase Agreement and the exhibits thereto,
and (ii) issued as a dividend or other distribution with respect to or in
exchange for or in replacement of the shares referenced in (i) above; PROVIDED,
HOWEVER, that Registrable Securities shall not include any shares of Tetra Tech
Common Stock that have previously been registered or sold to the public or have
been sold in a private transaction (excluding the issuance of the Tetra Tech
Common Stock pursuant to the Stock Purchase Agreement).

                           "REGISTRATION STATEMENT" shall mean any registration
statement of Tetra Tech in compliance with the Securities Act that covers
Registrable Securities pursuant to the provisions of this Agreement, including,
without limitation, the Prospectus, all amendments and supplements to such
Registration Statement, including all post-effective amendments, all exhibits
and all material incorporated by reference in such Registration Statement.

                           "RULE 144" shall mean Rule 144 promulgated under the
Securities Act or any similar successor rule, as the same shall be in effect
from time to time.

                           "RULE 144A" shall mean Rule 144A promulgated under
the Securities Act or any similar successor rule, as the same shall be in effect
from time to time.

                           "RULE 145" shall mean Rule 145 promulgated under the
Securities Act, or any similar successor rule, as the same shall be in effect
from time to time.

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                           "RULE 415" shall mean Rule 415 promulgated under the
Securities Act, or any similar successor rule, as the same shall be in effect
from time to time.

                           "SECURITIES ACT" shall mean the Securities Act of
1933, as amended from time to time.

                           "SEC" shall mean the Securities and Exchange
Commission.

                           "UNDERWRITTEN OFFERING" shall mean a registration in
which securities of Tetra Tech are sold to an underwriter or through an
underwriter as agent for reoffering to the public.

                  2.       REGISTRATION.

                           (a) Tetra Tech shall use all commercially reasonable
efforts to file a Registration Statement on Form S-1, Form S-2 or Form S-3,
whichever Tetra Tech shall be eligible to use, providing for the sale by the
Holders, pursuant to Rule 415, and/or any similar rule that may be adopted by
the SEC, of the Registrable Securities, to cause such Registration Statement to
become effective not later than November 3, 2000, and to keep such Registration
Statement continuously effective for a period ending on the date on which all
Holders are eligible to sell Registrable Securities under Rule 144 without any
volume limitation (or similar successor Rule).

                           (b) No Holder shall have the right to register
securities under this Agreement unless such Holder provides and/or confirms in
writing prior to or after the filing of the Registration Statement such
information (including, without limitation, information as to the number of
Registrable Securities that such Holder has sold pursuant to any such
Registration Statement from time to time) as Tetra Tech may reasonably request
in connection with such Registration Statement.

                           (c) Notwithstanding the foregoing, for a period not
to exceed 90 days, Tetra Tech shall not be obligated to prepare and file the
Registration Statement required hereunder if Tetra Tech, in its good faith
judgment, reasonably believes that the filing of such Registration Statement
would require the disclosure of material non-public information regarding Tetra
Tech and, accordingly, that the filing thereof, at the time requested, or the
offering of Tetra Tech Common Stock pursuant thereto, would materially and
adversely affect (i) a pending or scheduled public offering or private placement
of securities of Tetra Tech, (ii) an acquisition, merger, consolidation or
similar transaction by or of Tetra Tech, (iii) preexisting and continuing
negotiations, discussions or pending proposals with respect to any of the
foregoing transactions, or (iv) the financial condition of Tetra Tech in view of
the disclosure of any pending or threatened litigation, claim, assessment or
governmental investigation which might be required thereby.

                                       3

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                  In the event that Tetra Tech, in good faith, reasonably
believes that such conditions are continuing after such 90-day period, it may,
with the consent of the Holders of a majority of the Registrable Securities
subject (or to be subject) to the Registration Statement, which consent shall
not be unreasonably withheld, extend such 90-day period for an additional 30
days. Any further delay shall require the consent of the Holders of a majority
of such shares.

                  3. REGISTRATION PROCEDURES. In connection with Tetra Tech's
registration obligations pursuant to SECTION 2 hereof, Tetra Tech will use its
commercially reasonable efforts to effect such registration to permit the sale
of the Registrable Securities covered thereby in accordance with the intended
method or methods of disposition thereof, and pursuant thereto Tetra Tech will:

                           (a) prepare and file with the SEC a Registration
Statement with respect to such Registrable Securities and use its commercially
reasonable efforts to cause such Registration Statement to become effective;
PROVIDED that, before filing any Registration Statement or Prospectus or any
amendments or supplements thereto, Tetra Tech will furnish to the Holders of the
Registrable Securities covered by such Registration Statement and their counsel,
copies of all such documents proposed to be filed at least ten days prior
thereto, and Tetra Tech will not file any such Registration Statement or
amendment thereto or any Prospectus or any supplement thereto to which any such
Holder shall reasonably object within such ten day period; PROVIDED, FURTHER,
that Tetra Tech will not name or otherwise provide any information with respect
to any Holder in any Registration Statement or Prospectus without the express
written consent of such Holder, unless required to do so by the Securities Act
and the rules and regulations thereunder;

                           (b) prepare and file with the SEC such amendments,
post-effective amendments and supplements to the Registration Statement and the
Prospectus as may be necessary to comply with the provisions of the Securities
Act and the rules and regulations thereunder with respect to the disposition of
all securities covered by such Registration Statement;

                           (c) promptly notify the selling Holders (i) when the
Prospectus or any Prospectus supplement or post-effective amendment has been
filed, and, with respect to the Registration Statement or any post-effective
amendment, when the same has become effective, (ii) of any request by the SEC
for amendments or supplements to the Registration Statement or the Prospectus or
for additional information, (iii) of the issuance by the SEC of any stop order
suspending the effectiveness of the Registration Statement or the initiation of
any proceedings for that purpose, (iv) of the receipt by Tetra Tech of any
notification with respect to the suspension of the qualification of the
Registrable Securities for sale in any jurisdiction or the initiation or
threatening of any proceeding for such purpose and (v) of the happening of any
event which makes any statement made in the Registration Statement, the
Prospectus or any

                                       4

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document incorporated therein by reference untrue or which requires the making
of any changes in the Registration Statement, the Prospectus or any document
incorporated therein by reference in order to make the statements therein not
misleading in light of the circumstances then existing;

                           (d) make every commercially reasonable effort to
obtain the withdrawal of any order suspending the effectiveness of the
Registration Statement at the earliest possible moment;

                           (e) furnish to each selling Holder, without charge,
at least one signed copy of the Registration Statement and any post-effective
amendment thereto, including financial statements and schedules, all documents
incorporated therein by reference and all exhibits (including those incorporated
by reference);

                           (f) deliver to each selling Holder, without charge,
such reasonable number of conformed copies of the Registration Statement (and
any post-effective amendment thereto) and such number of copies of the
Prospectus (including each preliminary prospectus) and any amendment or
supplement thereto (and any documents incorporated by reference therein) as such
Holder may reasonably request; Tetra Tech consents to the use of the Prospectus
or any amendment or supplement thereto by each of the selling Holders in
connection with the offer and sale of the Registrable Securities covered by the
Prospectus or any amendment or supplement thereto;

                           (g) prior to any offering of Registrable Securities
covered by a Registration Statement, register or qualify or cooperate with the
selling Holders in connection with the registration or qualification of such
Registrable Securities for offer and sale under the securities or blue sky laws
of such jurisdictions as any such selling Holder reasonably requests, and use
its reasonable efforts to keep each such registration or qualification
effective, including through new filings, or amendments or renewals, during the
period such Registration Statement is required to be kept effective pursuant to
the terms of this Agreement; and do any and all other acts or things necessary
or advisable to enable the disposition in all such jurisdictions reasonably
requested by the Holders of the Registrable Securities covered by such
Registration Statement, PROVIDED that under no circumstances shall Tetra Tech be
required in connection therewith or as a condition thereof to qualify to do
business or to file a general consent to service of process in any such states
or jurisdictions;

                           (h) cooperate with the selling Holders to facilitate
the timely preparation and delivery of certificates representing Registrable
Securities to be sold, free of any and all restrictive legends, such
certificates to be in such denominations and registered in such names as the
Holders may request;

                           (i) upon the occurrence of any event contemplated by
SECTION 3(c)(v) above, prepare a supplement or post-effective amendment to the
Registration Statement or the

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Prospectus or any document incorporated therein by reference or file any other
required document so that, as thereafter delivered to the purchasers of the
Registrable Securities, the Prospectus will not contain an untrue statement of a
material fact or omit to state any material fact necessary to make the
statements therein, in light of the circumstances under which they were made,
not misleading;

                           (j) make generally available to the holders of Tetra
Tech's outstanding securities earnings statements satisfying the provisions of
Section 11(a) of the Securities Act, no later than 60 days after the end of any
12 month period (or 90 days, if such period is a fiscal year) beginning with the
first month of Tetra Tech's first fiscal quarter commencing after the effective
date of the Registration Statement, which statements shall cover said 12 month
period;

                           (k) provide and cause to be maintained a transfer
agent and registrar for all Registrable Securities covered by each Registration
Statement from and after a date not later than the effective date of such
Registration Statement;

                           (l) use its commercially reasonable efforts to cause
all Registrable Securities covered by each Registration Statement to be listed,
subject to notice of issuance, prior to the date of the first sale of such
Registrable Securities pursuant to such Registration Statement, on each
securities exchange on which the Tetra Tech Common Stock is then listed, and
admitted to trading on the Nasdaq Stock Market, if the Tetra Tech Common Stock
is then admitted to trading on the Nasdaq Stock Market; and

                           (m) enter into such agreements (including
underwriting agreements in customary form containing, among other things,
reasonable and customary indemnities) and take such other actions as a majority
of the Holders shall reasonably request in order to expedite or facilitate the
disposition of such Registrable Securities.

Each Holder agrees that, upon receipt of any notice from Tetra Tech of the
happening of any event of the kind described in SECTION 3(c)(v) hereof, such
Holder will forthwith discontinue disposition of Registrable Securities under
the Prospectus related to the applicable Registration Statement until such
Holder's receipt of the copies of the supplemented or amended Prospectus
contemplated by SECTION 3(i) hereof, or until it is advised in writing by Tetra
Tech that the use of the Prospectus may be resumed.

                  It shall be a condition precedent to the obligations of Tetra
Tech to take any action pursuant to this SECTION 3 with respect to the
Registrable Securities of any selling Holder that such Holder shall furnish to
Tetra Tech such information regarding itself and the Registrable Securities held
by it as shall be required by the Securities Act to effect the registration of
such Holder's Registrable Securities.

                                       6

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                  4. REGISTRATION EXPENSES. All expenses incident to any
registration to be effected hereunder and incident to Tetra Tech's performance
of or compliance with this Agreement, including without limitation all
registration and filing fees, fees and expenses of compliance with securities or
blue sky laws, printing expenses, messenger and delivery expenses, National
Association of Securities Dealers, Inc., stock exchange and qualification fees,
fees and disbursements of Tetra Tech's counsel and of independent certified
public accountants of Tetra Tech (including the expenses of any special audit
required by or incident to such performance), the fees of one counsel and one
accountant representing the Holders in such offering, expenses of the
underwriters that are customarily requested in similar circumstances by such
underwriters (excluding discounts, commissions or fees of underwriters, selling
brokers, dealer managers or similar securities industry professionals relating
to the distribution of the Registrable Securities, which will be borne by the
Holders), all such expenses being herein called "Registration Expenses," will be
borne by Tetra Tech. Tetra Tech will also pay its internal expenses, the expense
of any annual audit and the fees and expenses of any person retained by Tetra
Tech.

                  5.       INDEMNIFICATION.

                           (a) INDEMNIFICATION BY TETRA TECH. Tetra Tech agrees
to indemnify and hold harmless each Holder of Registrable Securities, its
officers, directors, partners and employees and each person who controls such
Holder (within the meaning of Section 15 of the Securities Act) from and against
any and all losses, claims, damages and liabilities (including any
investigation, legal or other expenses reasonably incurred in connection with,
and any amount paid in settlement of, any action, suit or proceeding or any
claim asserted) (collectively, "Damages") to which such Holder may become
subject under the Securities Act, the Exchange Act or other federal or state
securities law or regulation, at common law or otherwise, insofar as such
Damages arise out of or are based upon (i) any untrue statement or alleged
untrue statement of a material fact contained in any Registration Statement,
Prospectus or preliminary prospectus or any amendment or supplement thereto,
(ii) the omission or alleged omission to state therein a material fact required
to be stated therein or necessary to make the statements therein, in light of
the circumstances under which they were made, not misleading and (iii) any
violation or alleged violation by Tetra Tech of the Securities Act, the Exchange
Act or any state securities or blue sky laws in connection with the Registration
Statement, Prospectus or preliminary prospectus or any amendment or supplement
thereto, PROVIDED that Tetra Tech will not be liable to any Holder to the extent
that such Damages arise from or are based upon any untrue statement or omission
(y) based upon written information furnished to Tetra Tech by such Holder
expressly for the inclusion in such Registration Statement, and (z) made in any
Prospectus if such untrue statement or omission was corrected in an amendment or
supplement to such Prospectus and such Holder failed to deliver such amendment
or supplement prior to or concurrently with the sale of Registrable Securities
to the party asserting the claim underlying such Damages.

                           (b) INDEMNIFICATION BY HOLDER OF REGISTRABLE
SECURITIES. Each Holder of Registrable Securities whose Registrable Securities
are sold under a Prospectus which is a part

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of a Registration Statement agrees to indemnify and hold harmless Tetra Tech,
its directors and each officer who signed such Registration Statement and each
person who controls Tetra Tech (within the meaning of Section 15 of the
Securities Act), and each other Holder of Registrable Securities whose
Registrable Securities are sold under the Prospectus which is a part of such
Registration Statement (and such Holder's officers, directors and employees and
each person who controls such Holder within the meaning of Section 15 of the
Securities Act), under the same circumstances as the foregoing indemnity from
Tetra Tech to each Holder of Registrable Securities to the extent that such
losses, claims, damages, liabilities or actions arise out of or are based upon
any untrue statement of a material fact or omission of a material fact that was
made in the Prospectus, the Registration Statement, or any amendment or
supplement thereto, in reliance upon and in conformity with information relating
to such Holder furnished in writing to Tetra Tech by such Holder expressly for
use therein, PROVIDED that in no event shall the aggregate liability of any
selling Holder of Registrable Securities exceed the amount of the net proceeds
received by such Holder upon the sale of the Registrable Securities giving rise
to such indemnification obligation. Tetra Tech and the selling Holders shall be
entitled to receive indemnities from underwriters, selling brokers, dealer
managers and similar securities industry professionals participating in the
distribution, to the same extent as customarily furnished by such persons in
similar circumstances.

                           (c) CONDUCT OF INDEMNIFICATION PROCEEDINGS. Any
person entitled to indemnification hereunder will (i) give prompt notice to the
indemnifying party of any claim with respect to which it seeks indemnification
and (ii) permit such indemnifying party to assume the defense of such claim with
counsel reasonably satisfactory to the indemnified party; PROVIDED, HOWEVER,
that any person entitled to indemnification hereunder shall have the right to
employ separate counsel and to participate in the defense of such claim, but the
fees and expenses of such counsel shall be at the expense of such person and not
of the indemnifying party unless (A) the indemnifying party has agreed to pay
such fees or expenses, (B) the indemnifying party shall have failed to assume
the defense of such claim and employ counsel reasonably satisfactory to such
person or (C) in the reasonable judgment of such person and the indemnifying
party, based upon advice of their respective counsel, a conflict of interest may
exist between such person and the indemnifying party with respect to such claims
(in which case, if the person notifies the indemnifying party in writing that
such person elects to employ separate counsel at the expense of the indemnifying
party, the indemnifying party shall not have the right to assume the defense of
such claim on behalf of such person). If such defense is not assumed by the
indemnifying party, the indemnifying party will not be subject to any liability
for any settlement made without its consent (but such consent will not be
unreasonably withheld). No indemnified party will be required to consent to
entry of any judgment or enter into any settlement which does not include as an
unconditional term thereof the giving by all claimants or plaintiffs to such
indemnified party of a release from all liability in respect to such claim or
litigation. Any indemnifying party who is not entitled to, or elects not to,
assume the defense of a claim will not be obligated to pay the fees and expenses
of more than one counsel for all parties indemnified by such indemnifying party
with respect to such claim. As used in this SECTION 5(c),

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the terms "indemnifying party", "indemnified party" and other terms of similar
import are intended to include only Tetra Tech (and its officers, directors and
control persons as set forth above) on the one hand, and the Holders (and their
officers, directors, partners, employees, attorneys and control persons as set
forth above) on the other hand, as applicable.

                           (d) CONTRIBUTION. If for any reason the foregoing
indemnity is unavailable, then the indemnifying party shall contribute to the
amount paid or payable by the indemnified party as a result of such losses,
claims, damages, liabilities or expenses (i) in such proportion as is
appropriate to reflect the relative benefits received by the indemnifying party
on the one hand and the indemnified party on the other, or (ii) if the
allocation provided by CLAUSE (i) above is not permitted by applicable law or
provides a lesser sum to the indemnified party than the amount hereinafter
calculated, in such proportion as is appropriate to reflect not only the
relative benefits received by the indemnifying party on the one hand and the
indemnified party on the other but also the relative fault of the indemnifying
party and the indemnified party as well as any other relevant equitable
considerations. Notwithstanding the foregoing, no Holder shall be required to
contribute any amount in excess of the amount such Holder would have been
required to pay to an indemnified party if the indemnity under SECTION 5(b)
hereof was available. No person guilty of fraudulent misrepresentation (within
the meaning of Section 11(f) of the Securities Act) shall be entitled to
contribution from any person who was not guilty of such fraudulent
misrepresentation. The obligation of any person to contribute pursuant to this
SECTION 5(d) shall be several and not joint.

                           (e) TIMING OF PAYMENTS. An indemnifying party shall
make payments of all amounts required to be made pursuant to the foregoing
provisions of this SECTION 5 to or for the account of the indemnified party from
time to time promptly upon receipt of bills or invoices relating thereto or when
otherwise due or payable.

                           (f) SURVIVAL. The indemnity and contribution
agreements contained in this SECTION 7 shall remain in full force and effect,
regardless of any investigation made by or on behalf of a participating Holder,
its officers, directors, partners, attorneys, agents or any person, if any, who
controls such Holder as aforesaid, and shall survive the transfer of such
Registrable Securities by such Holder.

                  6.       PREPARATION; REASONABLE INVESTIGATION. In
connection with the preparation and filing of a Registration Statement
pursuant to the terms of this Agreement:

                           (a) Tetra Tech shall, with respect to a Registration
Statement filed pursuant to SECTION 2, give the Holders of such Registrable
Securities so registered, their underwriters, if any, and their respective
counsel and accountants the opportunity to participate in the preparation of
such Registration Statement (other than reports and proxy statements
incorporated therein by reference and lawfully and properly filed with the SEC)
and each

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Prospectus included therein or filed with the SEC, and each amendment thereof or
supplement thereto; and

                     (b) Tetra Tech shall give the Holders of such
Registrable Securities so registered, their underwriters, if any, and their
respective counsel and accountants such reasonable access to its books and
records and such opportunities to discuss the business of Tetra Tech with its
officers and the independent public accountants who have certified its
financial statements as shall be necessary, in the opinion of such Holders or
such underwriters, to conduct a reasonable investigation within the meaning
of Section 11(b)(3) of the Securities Act.

                  7. RULE 144. Tetra Tech covenants that it will use its best
efforts to file, on a timely basis, the reports required to be filed by it under
the Securities Act and the Exchange Act and the rules and regulations adopted by
the SEC thereunder, and it will take such further action as any Holder may
reasonably request (including, without limitation, compliance with the current
public information requirements of Rule 144(c) and Rule 144A), all to the extent
required from time to time to enable such Holder to sell Registrable Securities
without registration under the Securities Act within the limitation of the
conditions provided by Rule 144, Rule 144A or any similar rule or regulation
hereafter adopted by the SEC. Upon the request of any Holder, Tetra Tech will
deliver to such holder a written statement verifying that it has complied with
such information and requirements.

                  8. SPECIFIC PERFORMANCE. Each Holder, in addition to being
entitled to exercise all rights provided herein or granted by law, including
recovery of damages, will be entitled to specific performance of its rights
under this Agreement. Tetra Tech agrees that monetary damages would not be
adequate compensation for any loss incurred by reason of a breach by it of the
provisions of this Agreement and hereby agrees to waive the defense in any
action for specific performance that a remedy at law would be adequate.

                  9. NOTICES. All notices and other communications required or
permitted hereunder shall be in writing and shall be mailed by United States
first-class mail, postage prepaid, sent by facsimile or delivered personally by
hand or nationally recognized courier addressed (a) if to a Holder, as indicated
on the list of Holders attached hereto as SCHEDULE A, or at such other address
as such Holder or permitted assignee shall have furnished to Tetra Tech in
writing, or (b) if to Tetra Tech, at such address or facsimile number as Tetra
Tech shall have furnished to each Holder in writing. All such notices and other
written communications shall be effective on the date of mailing, facsimile
transfer or delivery.

                  10. SUCCESSORS AND ASSIGNS: ASSIGNMENT OF RIGHTS. The rights
and benefits of a Holder hereunder may not be assigned to a transferee or
assignee, without the consent of Tetra Tech; PROVIDED, HOWEVER, that, no later
than the 10th day prior to the filing of the Registration Statement under
SECTION 2 hereof, the rights and benefits of a Holder hereunder may be
transferred in connection with a transfer or assignment of any Registrable
Securities held by such

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Holder (i) by gift to immediate family members of such Holder, or trusts or
other entities for the sole benefit thereof, or (ii) by gift to any entity in
which such Holder, his or her immediate family members, or trusts or other
entities for the sole benefit thereof beneficially own all of the voting
securities; PROVIDED, HOWEVER, that in each case, the transferee executes an
instrument pursuant to which the transferee agrees to be bound by the terms and
conditions hereof as a Holder, and such other documents as Tetra Tech or its
counsel may reasonably require, after which, such transferee shall be deemed a
"Holder" hereunder. Any transfer of Registrable Securities, and rights
hereunder, shall be subject to compliance with applicable securities laws and
the restrictions contained in the Investment Letter executed by each Holder
pursuant to the Stock Purchase Agreement.

                  11. SEVERABILITY. In the event that any one or more of the
provisions contained herein, or the application thereof in any circumstance, is
held invalid, illegal or unenforceable, the validity, legality and
enforceability of any such provision in every other respect and of the remaining
provisions contained herein shall not be affected or impaired thereby.

                  12. ENTIRE AGREEMENT; AMENDMENT; WAIVER. This Agreement, the
Stock Purchase Agreement and the other agreements contemplated thereby
constitute the full and entire understanding and agreement among the parties
with regard to the subjects hereof and thereof. Without limiting the foregoing,
the rights of the Holders to registration pursuant to the terms of this
Agreement shall be subject to the limitations on resale contained in the
Investment Letter (as defined in the Stock Purchase Agreement). Neither this
Agreement nor any term hereof may be amended, waived, discharged or terminated,
except by a written instrument signed by Tetra Tech and the holders of at least
51% of the Registrable Securities and any such amendment, waiver, discharge or
termination shall be binding upon all the parties hereto, but in no event shall
the obligation of any party hereto be materially increased, except upon the
written consent of such party.

                  13. COUNTERPARTS. This Agreement may be executed in any number
of counterparts, each of which shall be original, and all of which together
shall constitute one instrument.

                  14. GOVERNING LAW. This Agreement shall be governed by, and
construed in accordance with, the laws of the State of California without giving
effect to principles of conflicts of laws thereof.

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                  15. NO THIRD PARTY BENEFICIARIES. The covenants and agreements
set forth herein are for the sole and exclusive benefit of the parties hereto
and their respective successors and assigns and such covenants and agreements
shall not be construed as conferring, and are not intended to confer, any rights
or benefits upon any other persons.

                  IN WITNESS WHEREOF, the parties hereto have executed this
Agreement as of the day and year first above written.

                                               TETRA TECH, INC.

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

                                        /s/ Emanuele Di Lecce
                                       -----------------------------------------
                                       Emanuele Di Lecce

                                        /s/ Brenda Di Lecce
                                       -----------------------------------------
                                       Brenda Di Lecce

                                       12

<PAGE>

<TABLE>
<CAPTION>
                                                                                                        SCHEDULE A

                                                 SCHEDULE OF HOLDERS

                                                                     Number of Shares of Tetra Tech Common
                                                                                Stock Issuable
                                                                            Upon Exchange Pursuant
         Holder's Name/Address/Facsimile No.                            to the Stock Purchase Agreement
-------------------------------------------------------       ----------------------------------------------------
<S>                                                           <C>
Emanuele Di Lecce                                                                   18,303

Brenda Di Lecce                                                                     15,303

</TABLE><PAGE>

                                                                   EXHIBIT 10.24

                          REGISTRATION RIGHTS AGREEMENT

                  This Registration Rights Agreement (the "Agreement") is
entered into as of May 17, 2000 by and among Tetra Tech, Inc., a Delaware
corporation ("Tetra Tech"), and the parties listed on SCHEDULE A attached hereto
(each, a "Holder" and collectively, the "Holders").

                                 R E C I T A L S

                  A. Tetra Tech and the Holders are parties to a Stock Purchase
Agreement dated as of the date hereof (the "Stock Purchase Agreement"); and

                  B. Pursuant to the Stock Purchase Agreement, the Holders will
receive shares of the common stock, $.01 par value, of Tetra Tech ("Tetra Tech
Common Stock"); and

                  C. This Agreement is the Registration Rights Agreement
referred to in SECTION 6.2 of the Stock Purchase Agreement and, pursuant
thereto, must be entered into by the parties in connection with the transactions
contemplated by the Stock Purchase Agreement.

                                A G R E E M E N T

                  NOW, THEREFORE, in consideration of the mutual covenants and
agreements contained herein, and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the parties hereto
agree as follows:

                  1. CERTAIN DEFINITIONS. As used in this Agreement, the
following terms shall have the following respective meanings:

                     "EXCHANGE ACT" shall mean the Securities Exchange Act of
1934, as amended from time to time.

                     "FORM S-3" shall mean such form under the Securities Act
as in effect on the date hereof or any successor registration form under the
Securities Act subsequently adopted by the SEC which permits inclusion or
incorporation of substantial information by reference to other documents
filed by Tetra Tech with the SEC.

                     "PROSPECTUS" shall mean the prospectus included in any
Registration Statement, as amended or supplemented by any prospectus
supplement, with respect to the terms of the offering of any portion of the
Registrable Securities covered by the Registration Statement and by all other
amendments and supplements to the prospectus, including post-effective
amendments and all material incorporated by reference in such Prospectus.

<PAGE>

                           "REGISTER", "REGISTERED" and "REGISTRATION" shall
mean and refer to a registration effected by preparing and filing a Registration
Statement and taking all other actions that are necessary or appropriate in
connection therewith, and the declaration or ordering of effectiveness of such
Registration Statement by the SEC.

                           "REGISTRATION EXPENSES" shall have the meaning set
forth in SECTION 6.

                           "REGISTRABLE SECURITIES" shall mean the shares of
Tetra Tech Common Stock (i) issued pursuant to the Stock Purchase Agreement and
the exhibits thereto, and (ii) issued as a dividend or other distribution with
respect to or in exchange for or in replacement of the shares referenced in (i)
above; PROVIDED, HOWEVER, that Registrable Securities shall not include any
shares of Tetra Tech Common Stock that have previously been registered or sold
to the public or have been sold in a private transaction (excluding the issuance
of the Tetra Tech Common Stock pursuant to the Stock Purchase Agreement).

                           "REGISTRATION STATEMENT" shall mean any registration
statement of Tetra Tech in compliance with the Securities Act that covers
Registrable Securities pursuant to the provisions of this Agreement, including,
without limitation, the Prospectus, all amendments and supplements to such
Registration Statement, including all post-effective amendments, all exhibits
and all material incorporated by reference in such Registration Statement.

                           "RULE 144" shall mean Rule 144 promulgated under the
Securities Act or any similar successor rule, as the same shall be in effect
from time to time.

                           "RULE 144A" shall mean Rule 144A promulgated under
the Securities Act or any similar successor rule, as the same shall be in effect
from time to time.

                           "RULE 145" shall mean Rule 145 promulgated under the
Securities Act, or any similar successor rule, as the same shall be in effect
from time to time.

                           "RULE 415" shall mean Rule 415 promulgated under the
Securities Act, or any similar successor rule, as the same shall be in effect
from time to time.

                           "SECURITIES ACT" shall mean the Securities Act of
1933, as amended from time to time.

                           "SEC" shall mean the Securities and Exchange
Commission.

                           "UNDERWRITTEN OFFERING" shall mean a registration in
which securities of Tetra Tech are sold to an underwriter or through an
underwriter as agent for reoffering to the public.

                                       2
<PAGE>

                  2.       REGISTRATION.

                           (a) Tetra Tech shall file a Registration Statement on
Form S-1, Form S-2 or Form S-3, whichever Tetra Tech shall be eligible to use,
providing for the sale by the Holders, pursuant to Rule 415, and/or any similar
rule that may be adopted by the SEC, of the Registrable Securities; to use its
reasonable best efforts to cause such Registration Statement to become effective
not later than November 17, 2000; and to use its reasonable best efforts to keep
such Registration Statement continuously effective for a period ending on the
date on which all Holders are eligible to sell Registrable Securities under Rule
144 without any volume limitation (or similar successor Rule).

                           (b) No Holder shall have the right to register
securities under this Agreement unless such Holder provides and/or confirms in
writing prior to or after the filing of the Registration Statement such
information (including, without limitation, information as to the number of
Registrable Securities that such Holder has sold pursuant to any such
Registration Statement from time to time) as Tetra Tech may reasonably request
in connection with such Registration Statement.

                           (c) Notwithstanding the foregoing, for a period not
to exceed 90 days, Tetra Tech shall not be obligated to prepare and file the
Registration Statement required hereunder if Tetra Tech, in its good faith
judgment, reasonably believes that the filing of such Registration Statement
would require the disclosure of material non-public information regarding Tetra
Tech and, accordingly, that the filing thereof, at the time requested, or the
offering of Tetra Tech Common Stock pursuant thereto, would materially and
adversely affect (i) a pending or scheduled public offering or private placement
of securities of Tetra Tech, (ii) an acquisition, merger, consolidation or
similar transaction by or of Tetra Tech, (iii) preexisting and continuing
negotiations, discussions or pending proposals with respect to any of the
foregoing transactions, or (iv) the financial condition of Tetra Tech in view of
the disclosure of any pending or threatened litigation, claim, assessment or
governmental investigation which might be required thereby.

                  In the event that Tetra Tech, in good faith, reasonably
believes that such conditions are continuing after such 90-day period, it may,
with the consent of the Holders of a majority of the Registrable Securities
subject (or to be subject) to the Registration Statement, which consent shall
not be unreasonably withheld, extend such 90-day period for an additional 30
days. Any further delay shall require the consent of the Holders of a majority
of such shares.

                  3. REGISTRATION PROCEDURES. In connection with Tetra Tech's
registration obligations pursuant to SECTION 2 hereof, Tetra Tech will use its
commercially reasonable efforts to effect such registration to permit the sale
of the Registrable Securities covered thereby in accordance with the intended
method or methods of disposition thereof, and pursuant thereto Tetra Tech will:

                                       3
<PAGE>

                           (a) prepare and file with the SEC a Registration
Statement with respect to such Registrable Securities and use its commercially
reasonable efforts to cause such Registration Statement to become effective;
PROVIDED that, before filing any Registration Statement or Prospectus or any
amendments or supplements thereto, Tetra Tech will furnish to the Holders of the
Registrable Securities covered by such Registration Statement and their counsel,
copies of all such documents proposed to be filed at least ten days prior
thereto, and Tetra Tech will not file any such Registration Statement or
amendment thereto or any Prospectus or any supplement thereto to which any such
Holder shall reasonably object within such ten day period; PROVIDED, FURTHER,
that Tetra Tech will not name or otherwise provide any information with respect
to any Holder in any Registration Statement or Prospectus without the express
written consent of such Holder, unless required to do so by the Securities Act
and the rules and regulations thereunder;

                           (b) prepare and file with the SEC such amendments,
post-effective amendments and supplements to the Registration Statement and the
Prospectus as may be necessary to comply with the provisions of the Securities
Act and the rules and regulations thereunder with respect to the disposition of
all securities covered by such Registration Statement;

                           (c) promptly notify the selling Holders (i) when the
Prospectus or any Prospectus supplement or post-effective amendment has been
filed, and, with respect to the Registration Statement or any post-effective
amendment, when the same has become effective, (ii) of any request by the SEC
for amendments or supplements to the Registration Statement or the Prospectus or
for additional information, (iii) of the issuance by the SEC of any stop order
suspending the effectiveness of the Registration Statement or the initiation of
any proceedings for that purpose, (iv) of the receipt by Tetra Tech of any
notification with respect to the suspension of the qualification of the
Registrable Securities for sale in any jurisdiction or the initiation or
threatening of any proceeding for such purpose and (v) of the happening of any
event which makes any statement made in the Registration Statement, the
Prospectus or any document incorporated therein by reference untrue or which
requires the making of any changes in the Registration Statement, the Prospectus
or any document incorporated therein by reference in order to make the
statements therein not misleading in light of the circumstances then existing;

                           (d) make every commercially reasonable effort to
obtain the withdrawal of any order suspending the effectiveness of the
Registration Statement at the earliest possible moment;

                           (e) furnish to each selling Holder, without charge,
at least one signed copy of the Registration Statement and any post-effective
amendment thereto, including financial statements and schedules, all documents
incorporated therein by reference and all exhibits (including those incorporated
by reference);

                                       4
<PAGE>

                           (f) deliver to each selling Holder, without charge,
such reasonable number of conformed copies of the Registration Statement (and
any post-effective amendment thereto) and such number of copies of the
Prospectus (including each preliminary prospectus) and any amendment or
supplement thereto (and any documents incorporated by reference therein) as such
Holder may reasonably request; Tetra Tech consents to the use of the Prospectus
or any amendment or supplement thereto by each of the selling Holders in
connection with the offer and sale of the Registrable Securities covered by the
Prospectus or any amendment or supplement thereto;

                           (g) prior to any offering of Registrable Securities
covered by a Registration Statement, register or qualify or cooperate with the
selling Holders in connection with the registration or qualification of such
Registrable Securities for offer and sale under the securities or blue sky laws
of such jurisdictions as any such selling Holder reasonably requests, and use
its reasonable efforts to keep each such registration or qualification
effective, including through new filings, or amendments or renewals, during the
period such Registration Statement is required to be kept effective pursuant to
the terms of this Agreement; and do any and all other acts or things necessary
or advisable to enable the disposition in all such jurisdictions reasonably
requested by the Holders of the Registrable Securities covered by such
Registration Statement, PROVIDED that under no circumstances shall Tetra Tech be
required in connection therewith or as a condition thereof to qualify to do
business or to file a general consent to service of process in any such states
or jurisdictions;

                           (h) cooperate with the selling Holders to facilitate
the timely preparation and delivery of certificates representing Registrable
Securities to be sold, free of any and all restrictive legends, such
certificates to be in such denominations and registered in such names as the
Holders may request;

                           (i) upon the occurrence of any event contemplated by
SECTION 3(c)(v) above, prepare a supplement or post-effective amendment to the
Registration Statement or the Prospectus or any document incorporated therein by
reference or file any other required document so that, as thereafter delivered
to the purchasers of the Registrable Securities, the Prospectus will not contain
an untrue statement of a material fact or omit to state any material fact
necessary to make the statements therein, in light of the circumstances under
which they were made, not misleading;

                           (j) make generally available to the holders of Tetra
Tech's outstanding securities earnings statements satisfying the provisions of
Section 11(a) of the Securities Act, no later than 60 days after the end of any
12 month period (or 90 days, if such period is a fiscal year) beginning with the
first month of Tetra Tech's first fiscal quarter commencing after the effective
date of the Registration Statement, which statements shall cover said 12 month
period;
                           (k) provide and cause to be maintained a transfer
agent and registrar for all Registrable Securities covered by each Registration
Statement from and after a date not later than the effective date of such
Registration Statement;

                                       5
<PAGE>

                           (l) use its commercially reasonable efforts to cause
all Registrable Securities covered by each Registration Statement to be listed,
subject to notice of issuance, prior to the date of the first sale of such
Registrable Securities pursuant to such Registration Statement, on each
securities exchange on which the Tetra Tech Common Stock is then listed, and
admitted to trading on the Nasdaq Stock Market, if the Tetra Tech Common Stock
is then admitted to trading on the Nasdaq Stock Market; and

                           (m) enter into such agreements (including
underwriting agreements in customary form containing, among other things,
reasonable and customary indemnities) and take such other actions as a majority
of the Holders shall reasonably request in order to expedite or facilitate the
disposition of such Registrable Securities.

Each Holder agrees that, upon receipt of any notice from Tetra Tech of the
happening of any event of the kind described in SECTION 3(c)(v) hereof, such
Holder will forthwith discontinue disposition of Registrable Securities under
the Prospectus related to the applicable Registration Statement until such
Holder's receipt of the copies of the supplemented or amended Prospectus
contemplated by SECTION 3(i) hereof, or until it is advised in writing by Tetra
Tech that the use of the Prospectus may be resumed.

                  It shall be a condition precedent to the obligations of Tetra
Tech to take any action pursuant to this SECTION 3 with respect to the
Registrable Securities of any selling Holder that such Holder shall furnish to
Tetra Tech such information regarding itself and the Registrable Securities held
by it as shall be required by the Securities Act to effect the registration of
such Holder's Registrable Securities.

                  4. REGISTRATION EXPENSES. All expenses incident to any
registration to be effected hereunder and incident to Tetra Tech's performance
of or compliance with this Agreement, including without limitation all
registration and filing fees, fees and expenses of compliance with securities or
blue sky laws, printing expenses, messenger and delivery expenses, National
Association of Securities Dealers, Inc., stock exchange and qualification fees,
fees and disbursements of Tetra Tech's counsel and of independent certified
public accountants of Tetra Tech (including the expenses of any special audit
required by or incident to such performance), the fees of one counsel and one
accountant representing the Holders in such offering, expenses of the
underwriters that are customarily requested in similar circumstances by such
underwriters (excluding discounts, commissions or fees of underwriters, selling
brokers, dealer managers or similar securities industry professionals relating
to the distribution of the Registrable Securities, which will be borne by the
Holders), all such expenses being herein called "Registration Expenses," will be
borne by Tetra Tech. Tetra Tech will also pay its internal expenses, the expense
of any annual audit and the fees and expenses of any person retained by Tetra
Tech.

                                       6
<PAGE>

                  5.       INDEMNIFICATION.

                           (a) INDEMNIFICATION BY TETRA TECH. Tetra Tech agrees
to indemnify and hold harmless each Holder of Registrable Securities, its
officers, directors, partners and employees and each person who controls such
Holder (within the meaning of Section 15 of the Securities Act) from and against
any and all losses, claims, damages and liabilities (including any
investigation, legal or other expenses reasonably incurred in connection with,
and any amount paid in settlement of, any action, suit or proceeding or any
claim asserted) (collectively, "Damages") to which such Holder may become
subject under the Securities Act, the Exchange Act or other federal or state
securities law or regulation, at common law or otherwise, insofar as such
Damages arise out of or are based upon (i) any untrue statement or alleged
untrue statement of a material fact contained in any Registration Statement,
Prospectus or preliminary prospectus or any amendment or supplement thereto,
(ii) the omission or alleged omission to state therein a material fact required
to be stated therein or necessary to make the statements therein, in light of
the circumstances under which they were made, not misleading and (iii) any
violation or alleged violation by Tetra Tech of the Securities Act, the Exchange
Act or any state securities or blue sky laws in connection with the Registration
Statement, Prospectus or preliminary prospectus or any amendment or supplement
thereto, PROVIDED that Tetra Tech will not be liable to any Holder to the extent
that such Damages arise from or are based upon any untrue statement or omission
(y) based upon written information furnished to Tetra Tech by such Holder
expressly for the inclusion in such Registration Statement, and (z) made in any
Prospectus if such untrue statement or omission was corrected in an amendment or
supplement to such Prospectus and such Holder failed to deliver such amendment
or supplement prior to or concurrently with the sale of Registrable Securities
to the party asserting the claim underlying such Damages.

                           (b) INDEMNIFICATION BY HOLDER OF REGISTRABLE
SECURITIES. Each Holder of Registrable Securities whose Registrable Securities
are sold under a Prospectus which is a part of a Registration Statement agrees
to indemnify and hold harmless Tetra Tech, its directors and each officer who
signed such Registration Statement and each person who controls Tetra Tech
(within the meaning of Section 15 of the Securities Act), and each other Holder
of Registrable Securities whose Registrable Securities are sold under the
Prospectus which is a part of such Registration Statement (and such Holder's
officers, directors and employees and each person who controls such Holder
within the meaning of Section 15 of the Securities Act), under the same
circumstances as the foregoing indemnity from Tetra Tech to each Holder of
Registrable Securities to the extent that such losses, claims, damages,
liabilities or actions arise out of or are based upon any untrue statement of a
material fact or omission of a material fact that was made in the Prospectus,
the Registration Statement, or any amendment or supplement thereto, in reliance
upon and in conformity with information relating to such Holder furnished in
writing to Tetra Tech by such Holder expressly for use therein, PROVIDED that in
no event shall the aggregate liability of any selling Holder of Registrable
Securities exceed the amount of the net proceeds received by such Holder upon
the sale of the Registrable Securities giving rise to such indemnification
obligation. Tetra Tech and the selling Holders shall be entitled to receive
indemnities from underwriters, selling brokers, dealer managers and similar
securities industry professionals participating in the distribution, to the same
extent as customarily furnished by such persons in similar circumstances.

                                       7
<PAGE>

                           (c) CONDUCT OF INDEMNIFICATION PROCEEDINGS. Any
person entitled to indemnification hereunder will (i) give prompt notice to the
indemnifying party of any claim with respect to which it seeks indemnification
and (ii) permit such indemnifying party to assume the defense of such claim with
counsel reasonably satisfactory to the indemnified party; PROVIDED, HOWEVER,
that any person entitled to indemnification hereunder shall have the right to
employ separate counsel and to participate in the defense of such claim, but the
fees and expenses of such counsel shall be at the expense of such person and not
of the indemnifying party unless (A) the indemnifying party has agreed to pay
such fees or expenses, (B) the indemnifying party shall have failed to assume
the defense of such claim and employ counsel reasonably satisfactory to such
person or (C) in the reasonable judgment of such person and the indemnifying
party, based upon advice of their respective counsel, a conflict of interest may
exist between such person and the indemnifying party with respect to such claims
(in which case, if the person notifies the indemnifying party in writing that
such person elects to employ separate counsel at the expense of the indemnifying
party, the indemnifying party shall not have the right to assume the defense of
such claim on behalf of such person). If such defense is not assumed by the
indemnifying party, the indemnifying party will not be subject to any liability
for any settlement made without its consent (but such consent will not be
unreasonably withheld). No indemnified party will be required to consent to
entry of any judgment or enter into any settlement which does not include as an
unconditional term thereof the giving by all claimants or plaintiffs to such
indemnified party of a release from all liability in respect to such claim or
litigation. Any indemnifying party who is not entitled to, or elects not to,
assume the defense of a claim will not be obligated to pay the fees and expenses
of more than one counsel for all parties indemnified by such indemnifying party
with respect to such claim. As used in this SECTION 5(c), the terms
"indemnifying party", "indemnified party" and other terms of similar import are
intended to include only Tetra Tech (and its officers, directors and control
persons as set forth above) on the one hand, and the Holders (and their
officers, directors, partners, employees, attorneys and control persons as set
forth above) on the other hand, as applicable.

                           (d) CONTRIBUTION. If for any reason the foregoing
indemnity is unavailable, then the indemnifying party shall contribute to the
amount paid or payable by the indemnified party as a result of such losses,
claims, damages, liabilities or expenses (i) in such proportion as is
appropriate to reflect the relative benefits received by the indemnifying party
on the one hand and the indemnified party on the other, or (ii) if the
allocation provided by CLAUSE (i) above is not permitted by applicable law or
provides a lesser sum to the indemnified party than the amount hereinafter
calculated, in such proportion as is appropriate to reflect not only the
relative benefits received by the indemnifying party on the one hand and the
indemnified party on the other but also the relative fault of the indemnifying
party and the indemnified party as well as any other relevant equitable
considerations. Notwithstanding the foregoing, no Holder shall be required to
contribute any amount in excess of the amount such Holder would have been
required to pay to an indemnified party if the indemnity under SECTION 5(b)
hereof was available. No person guilty of fraudulent misrepresentation (within
the meaning of Section 11(f) of the Securities Act) shall be entitled to
contribution from any person who was not guilty of such

                                       8
<PAGE>

fraudulent misrepresentation. The obligation of any person to contribute
pursuant to this SECTION 5(d) shall be several and not joint.

                           (e) TIMING OF PAYMENTS. An indemnifying party shall
make payments of all amounts required to be made pursuant to the foregoing
provisions of this SECTION 5 to or for the account of the indemnified party from
time to time promptly upon receipt of bills or invoices relating thereto or when
otherwise due or payable.

                           (f) SURVIVAL. The indemnity and contribution
agreements contained in this SECTION 5 shall remain in full force and effect,
regardless of any investigation made by or on behalf of a participating Holder,
its officers, directors, partners, attorneys, agents or any person, if any, who
controls such Holder as aforesaid, and shall survive the transfer of such
Registrable Securities by such Holder.

                  6. PREPARATION; REASONABLE INVESTIGATION. In connection with
the preparation and filing of a Registration Statement pursuant to the terms of
this Agreement:

                           (a) Tetra Tech shall, with respect to a Registration
Statement filed pursuant to SECTION 2, give the Holders of such Registrable
Securities so registered, their underwriters, if any, and their respective
counsel and accountants the opportunity to participate in the preparation of
such Registration Statement (other than reports and proxy statements
incorporated therein by reference and lawfully and properly filed with the SEC)
and each Prospectus included therein or filed with the SEC, and each amendment
thereof or supplement thereto; and

                           (b) Tetra Tech shall give the Holders of such
Registrable Securities so registered, their underwriters, if any, and their
respective counsel and accountants such reasonable access to its books and
records and such opportunities to discuss the business of Tetra Tech with its
officers and the independent public accountants who have certified its financial
statements as shall be necessary, in the opinion of such Holders or such
underwriters, to conduct a reasonable investigation within the meaning of
Section 11(b)(3) of the Securities Act.

                  7. RULE 144. Tetra Tech covenants that it will use its best
efforts to file, on a timely basis, the reports required to be filed by it under
the Securities Act and the Exchange Act and the rules and regulations adopted by
the SEC thereunder, and it will take such further action as any Holder may
reasonably request (including, without limitation, compliance with the current
public information requirements of Rule 144(c) and Rule 144A), all to the extent
required from time to time to enable such Holder to sell Registrable Securities
without registration under the Securities Act within the limitation of the
conditions provided by Rule 144, Rule 144A or any similar rule or regulation
hereafter adopted by the SEC. Upon the request of any Holder, Tetra Tech will
deliver to such holder a written statement verifying that it has complied with
such information and requirements.

                                       9
<PAGE>

                  8. SPECIFIC PERFORMANCE. Each Holder, in addition to being
entitled to exercise all rights provided herein or granted by law, including
recovery of damages, will be entitled to specific performance of its rights
under this Agreement. Tetra Tech agrees that monetary damages would not be
adequate compensation for any loss incurred by reason of a breach by it of the
provisions of this Agreement and hereby agrees to waive the defense in any
action for specific performance that a remedy at law would be adequate.

                  9. NOTICES. All notices and other communications required or
permitted hereunder shall be in writing and shall be mailed by United States
first-class mail, postage prepaid, sent by facsimile or delivered personally by
hand or nationally recognized courier addressed (a) if to a Holder, as indicated
on the list of Holders attached hereto as SCHEDULE A, or at such other address
as such Holder or permitted assignee shall have furnished to Tetra Tech in
writing, or (b) if to Tetra Tech, at such address or facsimile number as Tetra
Tech shall have furnished to each Holder in writing. All such notices and other
written communications shall be effective on the date of mailing, facsimile
transfer or delivery.

                  10. SUCCESSORS AND ASSIGNS: ASSIGNMENT OF RIGHTS. The rights
and benefits of a Holder hereunder may not be assigned to a transferee or
assignee, without the consent of Tetra Tech; PROVIDED, HOWEVER, that, no later
than the 10th day prior to the filing of the Registration Statement under
SECTION 2 hereof, the rights and benefits of a Holder hereunder may be
transferred in connection with a transfer or assignment of any Registrable
Securities held by such Holder (i) by gift to immediate family members of such
Holder, or trusts or other entities for the sole benefit thereof, or (ii) by
gift to any entity in which such Holder, his or her immediate family members, or
trusts or other entities for the sole benefit thereof beneficially own all of
the voting securities; PROVIDED, HOWEVER, that in each case, the transferee
executes an instrument pursuant to which the transferee agrees to be bound by
the terms and conditions hereof as a Holder, and such other documents as Tetra
Tech or its counsel may reasonably require, after which, such transferee shall
be deemed a "Holder" hereunder. Any transfer of Registrable Securities, and
rights hereunder, shall be subject to compliance with applicable securities laws
and the restrictions contained in the Investment Letter executed by each Holder
pursuant to the Stock Purchase Agreement.

                  11. SEVERABILITY. In the event that any one or more of the
provisions contained herein, or the application thereof in any circumstance, is
held invalid, illegal or unenforceable, the validity, legality and
enforceability of any such provision in every other respect and of the remaining
provisions contained herein shall not be affected or impaired thereby.

                  12. ENTIRE AGREEMENT; AMENDMENT; WAIVER. This Agreement, the
Stock Purchase Agreement and the other agreements contemplated thereby
constitute the full and entire understanding and agreement among the parties
with regard to the subjects hereof and thereof. Without limiting the foregoing,
the rights of the Holders to registration pursuant to the terms of this
Agreement shall be subject to the limitations on resale contained in the
Investment Letter (as defined in the Stock Purchase Agreement). Neither this
Agreement nor any term hereof may be

                                       10
<PAGE>

amended, waived, discharged or terminated, except by a written instrument signed
by Tetra Tech and the holders of at least 51% of the Registrable Securities and
any such amendment, waiver, discharge or termination shall be binding upon all
the parties hereto, but in no event shall the obligation of any party hereto be
materially increased, except upon the written consent of such party.

                  13. COUNTERPARTS. This Agreement may be executed in any number
of counterparts, each of which shall be original, and all of which together
shall constitute one instrument.

                  14. GOVERNING LAW. This Agreement shall be governed by, and
construed in accordance with, the laws of the State of California without giving
effect to principles of conflicts of laws thereof.

                                       11
<PAGE>

                  15. NO THIRD PARTY BENEFICIARIES. The covenants and agreements
set forth herein are for the sole and exclusive benefit of the parties hereto
and their respective successors and assigns and such covenants and agreements
shall not be construed as conferring, and are not intended to confer, any rights
or benefits upon any other persons.

                  IN WITNESS WHEREOF, the parties hereto have executed this
Agreement as of the day and year first above written.

                                               TETRA TECH, INC.

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

                                       /s/ Anthony R. Dark
                                       -----------------------------------------
                                       Anthony R. Dark

                                       /s/ Barry L. Burks
                                       -----------------------------------------
                                       Barry L. Burks

                                       /s/ Jon D. Nelson
                                       -----------------------------------------
                                       Jon D. Nelson

                                       12
<PAGE>

<TABLE>
<CAPTION>

                                                                                                     SCHEDULE A

                                            SCHEDULE OF HOLDERS

                                                               Number of Shares of Tetra Tech Common
                                                                       Stock Issuable Pursuant
         Holder's Name/Address/Facsimile No.                      to the Stock Purchase Agreement
--------------------------------------------------      --------------------------------------------------
<S>                                                     <C>
Anthony R. Dark                                                               17,669
c/o FHC, Inc.
5416 South Yale Avenue
Tulsa, Oklahoma  74135

Barry L. Burks                                                                17,669
c/o FHC, Inc.
5416 South Yale Avenue
Tulsa, Oklahoma  74135

Jon D. Nelson                                                                 17,669
c/o FHC, Inc.
5416 South Yale Avenue
Tulsa, Oklahoma  74135

</TABLE>

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