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EXHIBIT 10.20

INDEMNITY AGREEMENT

        THIS INDEMNITY AGREEMENT (this "Agreement") dated as
of                        , 2004, is made by and between Tetra Tech, Inc., a
Delaware corporation (the "Company"), and                        (the
"Indemnitee").

R E C I T A L S:

        A.    The Company recognizes that competent and experienced persons are
increasingly reluctant to serve as directors and officers of corporations unless
they are protected by comprehensive liability insurance or indemnification, or
both, due to increased exposure to litigation costs and risks resulting from
their service to such corporations, and due to the fact that the exposure
frequently bears no reasonable relationship to the compensation of such
directors or officers.

        B.    The statutes and judicial decisions regarding the duties of
directors and officers are often difficult to apply, ambiguous or conflicting,
and therefore fail to provide such directors with adequate, reliable knowledge
of legal risks to which they are exposed or information regarding the proper
course of action to take.

        C.    The Company and the Indemnitee recognize that plaintiffs often
seek damages in such large amounts and the costs of litigation may be so
substantial (whether or not the case is meritorious), that the defense and/or
settlement of such litigation is often beyond the personal resources of
directors and officers.

        D.    The Company believes that it is unfair for its directors and
officers to assume the risk of substantial judgments and other expenses which
may occur in cases in which the director and/or officer, as the case may be,
received no personal profit and in cases where such person acted in good faith.

        E.    Section 145 of the General Corporation Law of Delaware
("Section 145"), under which the Company is organized, empowers the Company to
indemnify its directors and officers by agreement and to indemnify persons who
serve, at the request of the Company, as the directors and officers of other
corporations or enterprises, and expressly provides that the indemnification
provided by Section 145 is not exclusive.

        F.    The Board of Directors of the Company has determined that
contractual indemnification as set forth herein is not only reasonable and
prudent but necessary to promote the best interests of the Company and its
stockholders.

        G.    The Company desires and has requested the Indemnitee to serve or
continue to serve as a director and/or officer of the Company.

        H.    The Indemnitee only is willing to serve, or to continue to serve,
as a director and/or officer of the Company if the Indemnitee is furnished the
indemnity provided for herein by the Company.

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A G R E E M E N T:

        NOW THEREFORE, in consideration of the mutual covenants and agreements
set forth below, the parties hereto, intending to be legally bound, hereby agree
as follows:

        1.    Definitions.

        (a)    Agent. For purposes of this Agreement, "agent" of the Company
means any person who: (i) is or was a director and/or officer of the Company or
a subsidiary of the Company; or (ii) is or was serving at the request of, for
the convenience of, or to represent the interest of the Company or a subsidiary
of the Company as a director, officer or trustee of, or in any other capacity
with, another foreign or domestic corporation, partnership, joint venture or
employee benefit plan.

        (b)    Expenses. For purposes of this Agreement, "expenses" includes all
direct and indirect costs of any type or nature whatsoever (including, without
limitation, all attorneys' fees and related disbursements, other out-of-pocket
costs and reasonable compensation for time spent by the Indemnitee for which he
is not otherwise compensated by the Company or any third party, provided that
the rate of compensation and estimated time involved is approved in advance by
the Board of Directors of the Company), actually and reasonably incurred by the
Indemnitee in connection with either the investigation, defense or appeal of a
proceeding or establishing or enforcing a right to indemnification under this
Agreement, Section 145 or otherwise, and amounts paid in settlement by or on
behalf of the Indemnitee, but shall not include any judgments, fines or
penalties actually levied against the Indemnitee.

        (c)    Proceedings. For the purposes of this Agreement, "proceeding"
means any threatened, pending, or completed action, suit, arbitration, hearing
or other proceeding, whether civil, criminal, administrative, investigative or
any other type whatsoever.

        (d)    Subsidiary. For purposes of this Agreement, "subsidiary" means
any corporation of which more than 50% of the outstanding voting securities are
owned directly or indirectly by the Company, by the Company and one or more
other subsidiaries, or by one or more other subsidiaries.

        2.    Agreement to Serve. The Indemnitee agrees to serve and/or continue
to serve as an agent of the Company or a subsidiary of the Company, at the will
of such corporation (or under separate agreement, if such agreement exists), in
the capacity the Indemnitee currently serves as an agent of such corporation, so
long as the Indemnitee is duly appointed or elected and qualified in accordance
with the applicable provisions of the Bylaws of such corporation or of any
subsidiary thereof, or until such time as the Indemnitee tenders his resignation
in writing; provided, however, that nothing contained in this Agreement is
intended to create any right to continued employment of the Indemnitee in any
capacity or to give the Company the right to require the Indemnitee to resign
from the Board of Directors of the Company. This Agreement shall continue in
force after the Indemnitee has ceased to serve as a director or officer of the
Company.

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        3.    Indemnification.

        (a)    Indemnification in Third Party Proceedings. Subject to Section 10
below, the Company shall indemnify the Indemnitee if the Indemnitee is a party
to or threatened to be made a party to or otherwise involved in any proceeding
(other than a proceeding by or in the name of the Company to procure a judgment
in its favor) by reason of the fact that the Indemnitee is or was an agent of
the Company, or by reason of any act or inaction by him in any such capacity,
against any and all expenses and liabilities of any type whatsoever (including,
but not limited to, judgments, fines and penalties), actually and reasonably
incurred by him in connection with the investigation, defense, settlement or
appeal of such proceeding, but only if the Indemnitee acted in good faith and in
a manner he reasonably believed to be in, or not opposed to, the best interests
of the Company, and, with respect to any criminal action or proceeding, had no
reasonable cause to believe Indemnitee's conduct was unlawful, pursuant to the
presumption set forth in subsection (c) below, as applicable. The termination of
any proceeding by judgment, order of court, settlement, conviction or on plea of
nolo contendere, or its equivalent, shall not, of itself, create a presumption
that the Indemnitee did not act in good faith in a manner which he reasonably
believed to be in, or not opposed to, the best interests of the Company, and
with respect to any criminal proceedings, that such person had reasonable cause
to believe that his conduct was unlawful.

        (b)    Indemnification in Derivative Actions. Subject to Section 10
below, the Company shall indemnify the Indemnitee if the Indemnitee is a party
to or threatened to be made a party to or otherwise involved in any proceeding
by or in the name of the Company to procure a judgment in its favor by reason of
the fact that the Indemnitee is or was an agent of the Company, or by reason of
any act or inaction by him in any such capacity, against all expenses actually
and reasonably incurred by the Indemnitee in connection with the investigation,
defense, settlement, or appeal of such proceedings, but only if the Indemnitee
acted in good faith and in a manner he reasonably believed to be in, or not
opposed to, the best interests of the Company, pursuant to the presumption set
forth in subsection (c) below; provided, however, that no indemnification under
this Section 3 shall be made in respect of any claim, issue or matter as to
which the Indemnitee shall have been finally adjudged to be liable to the
Company by a court of competent jurisdiction due to willful misconduct of a
culpable nature in the performance of the Indemnitee's duty to the Company,
unless and only to the extent that any court in which such proceeding was
brought shall determine upon application that, despite the adjudication of
liability but in view of all the circumstances of the case, such person is
fairly and reasonably entitled to indemnity for such expenses as such court
shall deem proper.

        (c)    Conclusive Presumption Regarding Indemnitee Conduct. With respect
to Sections 3(a) and 3(b) above, the Indemnitee shall be conclusively presumed
to have acted in good faith and in a manner Indemnitee reasonably believed to be
in, or not opposed to, the best interests of the Company, and, with respect to
any criminal action or proceeding, to have had no reasonable cause to believe
Indemnitee's conduct was unlawful, unless a determination is made that the
Indemnitee has not acted in accordance with the standards set forth above (i) by
the Board of Directors by a majority vote of a quorum thereof consisting of
directors who were not parties to the proceeding due to which a claim is made
under this Agreement, (ii) by the stockholders of the Company by a majority vote
of stockholders who were not parties to such a proceeding, or (iii) in a written
opinion of independent legal counsel, selection of whom has been approved by the

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Indemnitee in writing or, if not so approved, then approved by a panel of
arbitrators, one of whom is selected by the Company, another of whom is selected
by the Indemnitee and the last of whom is selected by the first two arbitrators
so selected.

        4.    Indemnification of Expenses of Successful Party. Notwithstanding
any other provisions of this Agreement, to the extent that the Indemnitee has
been successful on the merits or otherwise in defense of any proceeding or in
defense of any claim, issue or matter therein, including the dismissal of any
action without prejudice, the Company shall indemnify the Indemnitee against all
expenses actually incurred in connection with the investigation, defense or
appeal of such proceeding.

        5.    Partial Indemnification. If the Indemnitee is entitled under any
provision of this Agreement to indemnification by the Company for some or a
portion of any expenses or liabilities of any type whatsoever (including, but
not limited to, judgments, fines or penalties) actually incurred by him in the
investigation, defense, settlement or appeal of a proceeding but is not
entitled, however, to indemnification for the total amount thereof, the Company
shall nevertheless indemnify the Indemnitee for the portion thereof to which the
Indemnitee is entitled.

        6.    Advancement of Expenses. Subject to Section 10(b) below, the
Company shall advance all expenses incurred by the Indemnitee in connection with
the investigation, defense, settlement or appeal of any proceeding to which the
Indemnitee is a party or is threatened to be made a party by reason of the fact
that the Indemnitee is or was an agent of the Company. The Indemnitee hereby
undertakes to repay such amounts advanced only if, and to the extent that, it
shall ultimately be determined that the Indemnitee is not entitled to be
indemnified by the Company as authorized by this Agreement. The advances to be
made hereunder shall be paid by the Company to or on behalf of the Indemnitee
within 30 days following delivery of a written request therefor by the
Indemnitee to the Company.

        7.    Notice and Other Indemnification Procedures.

        (a)    Notification of Proceeding. Promptly after receipt by the
Indemnitee of notice of the commencement of or the threat of commencement of any
proceeding, the Indemnitee shall, if the Indemnitee believes that
indemnification with respect thereto may be sought from the Company under this
Agreement, notify the Company of the commencement or threat of commencement
thereof.

        (b)    Request for Indemnification. Any indemnification requested by the
Indemnitee under Section 3 hereof shall be made no later than 10 days after
receipt of the written request of the Indemnitee, unless a good faith
determination is made by the Company's Board of Directors within said 10-day
period in accordance with one of the methods set forth in Section 3(c) above
that the Indemnitee is not or (subject to final judgment or other final
adjudication as provided in Section 10(a) below) ultimately will not be entitled
to indemnification hereunder.

        (c)    Application for Enforcement. Notwithstanding a determination
under Section 7(b) above that the Indemnitee is not entitled to indemnification
with respect to any specific proceeding, the Indemnitee shall have the right to
apply to any court of competent jurisdiction for the

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purpose of enforcing the Indemnitee's right to indemnification pursuant to this
Agreement. In such an enforcement hearing or proceeding, the burden of proving
by clear and convincing evidence that indemnification or advances are not
appropriate shall be on the Company. Neither the failure of the Company
(including its Board of Directors, stockholders, independent legal counsel or
the panel of arbitrators) to have made a determination prior to the commencement
of such action that the Indemnitee is entitled to indemnification hereunder, nor
an actual determination by the Company (including its Board of Directors or
independent legal counsel or the panel of arbitrators) that the Indemnitee is
not entitled to indemnification hereunder, shall be a defense to the action or
create any presumption that the Indemnitee is not entitled to indemnification
hereunder.

        (d)    Indemnification of Certain Expenses. The Company shall indemnify
the Indemnitee against all expenses incurred in connection with any hearing or
proceeding under this Section 7 unless the Company prevails by clear and
convincing evidence in such hearing or proceeding.

        8.    Assumption of Defense. In the event the Company shall be obligated
to pay the expenses of any proceeding against the Indemnitee, the Company, if
appropriate, shall be entitled to assume the defense of such proceeding, with
counsel reasonably acceptable to the Indemnitee, upon the delivery to the
Indemnitee of written notice of its election to do so. After delivery of such
notice, approval of such counsel by the Indemnitee and the retention of such
counsel by the Company, the Company shall not be liable to the Indemnitee under
this Agreement for any fees of counsel subsequently incurred by the Indemnitee
with respect to the same proceeding, provided that (a) the Indemnitee shall have
the right to employ his counsel in such proceeding at the Indemnitee's expense;
and (b) if (i) the employment of counsel by the Indemnitee has been previously
authorized in writing by the Company, (ii) the Indemnitee's counsel delivers a
written notice to the Company stating that such counsel has reasonably concluded
that there may be a conflict of interest between the Company and the Indemnitee
in the conduct of any such defense or (iii) the Company shall not, in fact, have
employed counsel to assume the defense of such proceeding within a reasonable
time, not exceeding 60 days or, if shorter, such period as shall not prejudice
the defense of the Indemnitee, then in any such event the fees and expenses of
the Indemnitee's counsel shall be at the expense of the Company.

        9.    Insurance. The Company shall maintain directors' and officers'
liability insurance ("D&O Insurance") with respect to which the Indemnitee is
named as an insured. Notwithstanding any other provision of this Agreement, the
Company shall not be obligated to indemnify the Indemnitee for expenses,
judgments, fines or penalties which have been paid directly to the Indemnitee by
D&O Insurance. If commercially available, the D&O Insurance shall provide for
coverage in the minimum amount of $20,000,000. The Company shall not take any
action intended to change the terms of the D&O Insurance or the rights of the
insureds granted thereunder without providing notice to, and obtaining the
approval of, the Board of Directors. At the time the Company receives from the
Indemnitee any notice of the commencement of a proceeding, the Company shall
give prompt notice of the commencement of such proceeding to the insurers in
accordance with the procedures set forth in the policy. The Company shall
thereafter take all necessary or desirable action to cause such insurers to pay,
on behalf of the Indemnitee, all amounts payable as a result of such proceeding
in accordance with the terms of such policy.

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        10.    Exceptions.

        (a)    Certain Matters. Any provision herein to the contrary
notwithstanding, the Company shall not be obligated pursuant to the terms of
this Agreement to indemnify the Indemnitee on account of any proceeding (i) with
respect to remuneration paid to the Indemnitee if it is determined by final
judgment or other final adjudication, in either case not subject to further
appeal, that such remuneration was in violation of law; (ii) which final
judgment is rendered against the Indemnitee for an accounting of profits made
from the purchase or sale by the Indemnitee of securities of the Company
pursuant to the provisions of Section 16(b) of the Securities Exchange Act of
1934, as amended; or (iii) which (but only to the extent that it) is determined
by final judgment or other final adjudication, in either case not subject to
further appeal, that the Indemnitee's conduct was in bad faith, knowingly
fraudulent or deliberately dishonest. For purposes of the foregoing sentence, a
final judgment or other adjudication may be reached in either the underlying
proceeding or action in connection with which indemnification is sought or a
separate proceeding or action to establish rights and liabilities under this
Agreement.

        (b)    Claims Initiated by the Indemnitee. Any provision herein to the
contrary notwithstanding, the Company shall not be obligated pursuant to the
terms of this Agreement to indemnify or advance expenses to the Indemnitee with
respect to proceedings or claims initiated or brought voluntarily by the
Indemnitee and not by way of defense, except with respect to proceedings brought
to establish or enforce a right to indemnification under this Agreement or any
other statute or law or otherwise as required under Section 145, but such
indemnification or advancement of expenses may be provided by the Company in
specific cases if the Board of Directors of the Company finds it to be
appropriate.

        (c)    Action for Indemnification. Any provision herein to the contrary
notwithstanding, the Company shall be obligated pursuant to the terms of this
Agreement to indemnify the Indemnitee for any expenses incurred by the
Indemnitee with respect to any proceeding instituted by the Indemnitee to
enforce or interpret this Agreement unless the Company prevails in such
proceeding by clear and convincing evidence.

        (d)    Unauthorized Settlements. Any provision herein to the contrary
notwithstanding, the Company shall not be obligated pursuant to the terms of
this Agreement to indemnify the Indemnitee under this Agreement for any amounts
paid in settlement of a proceeding effected without the Company's written
consent. Neither the Company nor the Indemnitee shall unreasonably withhold
consent to any proposed settlement; provided, however, that the Company may in
any event decline to consent to (or to otherwise admit or agree to any liability
for indemnification hereunder in respect of) any proposed settlement if the
Company determines in good faith (pursuant to Section 7(b) above) that the
Indemnitee is not or ultimately will not be entitled to indemnification
hereunder.

        (e)    Securities Act Liabilities. Any provision herein to the contrary
notwithstanding, the Company shall not be obligated pursuant to the terms of
this Agreement to indemnify the Indemnitee or otherwise act in violation of any
undertaking appearing in and required by the rules and regulations promulgated
under the Securities Act of 1933, as amended (the "Act") in any registration
statement filed with the Securities and Exchange Commission under the Act.

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The Indemnitee acknowledges that paragraph (h) of Item 512 of Regulation S-K
currently generally requires the Company to undertake in connection with any
registration statement filed under the Act to submit the issue of the
enforceability of the Indemnitee's rights under this Agreement in connection
with any liability under the Act on public policy grounds to a court of
appropriate jurisdiction and to be governed by any final adjudication of such
issue. The Indemnitee specifically agrees that any such undertaking shall
supersede the provisions of this Agreement and to be bound by any such
undertaking.

        11.    Nonexclusivity. The provisions for indemnification and
advancement of expenses set forth in this Agreement shall not be deemed
exclusive of any other rights which the Indemnitee may have under any provision
of law, the Company's Certificate of Incorporation or Bylaws, in any court in
which a proceeding is brought, the vote of the Company's stockholders or
disinterested directors, other agreements or otherwise, both as to action in the
Indemnitee's official capacity and to action in another capacity while occupying
his position as an agent of the Company, and the Indemnitee's rights hereunder
shall continue after the Indemnitee has ceased acting as an agent of the Company
and shall inure to the benefit of the heirs, executors and administrators of the
Indemnitee. Any provision herein to the contrary notwithstanding, the Company
may provide, in specific cases, the Indemnitee with full or partial
indemnification if the Board of Directors of the Company determines that such
indemnification is appropriate.

        12.    Subrogation. In the event of payment under this Agreement, the
Company shall be subrogated to the extent of such payment to all of the rights
of recovery of the Indemnitee, who, at the request and expense of the Company,
shall execute all papers required and shall do everything that may be reasonably
necessary to secure such rights, including the execution of such documents as
may be necessary to enable the Company effectively to bring suit to enforce such
rights.

        13.    Interpretation of Agreement. It is understood that the parties
hereto intend this Agreement to be interpreted and enforced so as to provide
indemnification to the Indemnitee to the fullest extent now or hereafter
permitted by law.

        14.    Severability. If any provision or provisions of this Agreement
shall be held to be invalid, illegal or unenforceable for any reason whatsoever,
(a) the validity, legality and enforceability of the remaining provisions of the
Agreement (including without limitation, all portions of any paragraphs of this
Agreement containing any such provision held to be invalid, illegal or
unenforceable, that are not themselves invalid, illegal or unenforceable) shall
not in any way be affected or impaired thereby; and (b) to the fullest extent
possible, the provisions of this Agreement (including, without limitation, all
portions of any paragraph of this Agreement containing any such provision held
to be invalid, illegal or unenforceable, that are not themselves invalid,
illegal or unenforceable) shall be construed so as to give effect to the intent
manifested by the provision held invalid, illegal or unenforceable and to give
effect to Section 13 hereof.

        15.    Modification and Waiver. No supplement, modification or amendment
of this Agreement shall be binding unless executed in writing by the parties
hereto. No waiver of any of the provisions of this Agreement shall be deemed or
shall constitute a waiver of any other provision hereof (whether or not similar)
nor shall such waiver constitute a continuing waiver.

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The indemnification rights afforded to the Indemnitee hereby are contract rights
and may not be diminished, eliminated or otherwise affected by amendments to the
Certificate of Incorporation or Bylaws of the Company or by other agreements.

        16.    Successors and Assigns. The terms of this Agreement shall bind,
and shall inure to the benefit of, the successors and assigns of the parties
hereto, including, without limitation, the surviving corporation in any merger
transaction or other business combination or reorganization to which the Company
is a party.

        17.    Duration of Agreement. This Agreement shall continue until and
terminate upon the later of: (i) ten years after the date that Indemnitee shall
have ceased to serve as a director or officer of the Company; or (ii) the final
termination of any proceeding then pending in respect of which the Indemnitee is
grated rights of indemnification or advance of expenses hereunder and of any
proceeding commenced by the Indemnitee pursuant to Section 7 hereof relating
thereto.

        18.    Counterparts. This Agreement may be executed in one or more
counterparts, each of which shall for all purposes be deemed to be an original
but all of which together shall constitute one and the same Agreement. Only one
such counterpart signed by the party against whom enforceability is sought needs
to be produced to evidence the existence of this Agreement.

        19.    Enforcement.

        (a)    Reliance. The Company expressly confirms and agrees that it has
entered into this Agreement and assumed the obligations imposed on it hereby in
order to induce the Indemnitee to serve or continue to serve as a director
and/or officer of the Company or its subsidiaries, and to serve upon any
committee of the Board of Directors of the Company or its subsidiaries as
requested by such Board, and the Company acknowledges that the Indemnitee is
relying upon this Agreement in serving as a director and/or officer of the
Company or its subsidiaries and a member of any such committee.

        (b)    Entire Agreement. This Agreement constitutes the entire agreement
between the parties hereto with respect to the subject matter hereof and
supersedes all prior agreements and understandings, oral, written and implied,
between the parties hereto with respect to the subject matter hereof.

        20.    Contribution. To the fullest extent permissible under applicable
law, if the indemnification provided for in this Agreement is unavailable to the
Indemnitee for any reason whatsoever, the Company, in lieu of indemnifying the
Indemnitee, shall contribute to the amount incurred by the Indemnitee, whether
for judgments, fines, penalties, excise taxes, amounts paid or to be paid in
settlement and/or for expenses, in connection with any claim relating to an
indemnifiable event under this Agreement, in such proportion as is deemed fair
and reasonable in light of all of the circumstances of such proceeding in order
to reflect (i) the relative benefits received by the Company and the Indemnitee
as a result of the event(s) and/or transaction(s) giving rise to such
proceeding; and (ii) the relative fault of the Company (and its directors,
officers, employees and agents) and the Indemnitee in connection with such
event(s) and/or transaction(s).

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        21.    Notice. Except as otherwise provided herein, any notice or demand
which, by the provisions hereof, is required or which may be given to or served
upon the parties hereto shall be in writing and, if by telegram, telecopy or
telex, shall be deemed to have been validly served, given or delivered when
sent, if by personal delivery, shall be deemed to have been validly served,
given or delivered upon actual delivery and, if mailed, shall be deemed to have
been validly served, given or delivered three business days after deposit in the
United States mails, as registered or certified mail, with proper postage
prepaid and addressed to the party or parties to be notified at the addresses
set forth on the signature page of this Agreement (or such other address(es) as
a party may designate for itself by like notice).

        22.    Governing Law. This Agreement shall be governed exclusively by
and construed according to the laws of the State of Delaware, as applied to
contracts between Delaware residents entered into and to be performed entirely
within Delaware.

        IN WITNESS WHEREOF, the parties hereto have entered into this Agreement
effective as of the date first above written.

    THE COMPANY:
 
 
TETRA TECH, INC.
a Delaware corporation
 
 
By:

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    Name: Li-San Hwang     Title: Chairman and Chief Executive Officer         
      Address: 3475 East Foothill Boulevard
Pasadena, California 91107
 
 
THE INDEMNITEE:
 
 

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Signature of the Indemnitee
 
 

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Print or Type Name of the Indemnitee
 
 
Address:
 

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QuickLinks

EXHIBIT 10.20

INDEMNITY AGREEMENT