Document:

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                                                                   Exhibit 10.17

                               AMENDMENT NO. 1 TO
                           FIFTH AMENDED AND RESTATED
                             STOCKHOLDERS' AGREEMENT

     This Amendment No. 1 to the Fifth Amended and Restated Stockholders'
Agreement (this "Amendment") is made and entered into as of July 12, 2006, by
and among Metabolix, Inc., a corporation organized under the laws of the State
of Delaware (the "Company"), the "Investor" identified on the signature page
hereto (the "Investor") and certain other stockholders of the Company set forth
on the signature page hereto (the "Stockholders").

     WHEREAS, the Company and the Stockholders are parties to that certain Fifth
Amended and Restated Stockholders' Agreement dated as of January 19, 2006 (the
"Fifth Amended Stockholders' Agreement");

     WHEREAS, the Investor is acquiring shares of the Company's common stock,
$.01 par value per share (the "Common Stock") pursuant to that certain Stock
Purchase Agreement of even date herewith (the "Purchase Agreement") by and
between the Company and the Investor;

     WHEREAS, pursuant to Section 6.4 of the Fifth Amended Stockholders'
Agreement, the Fifth Amended Stockholders' Agreement may be amended upon the
written consent of the Company and each of Edward M. Giles, State Farm Mutual
Automobile Insurance Company, Vertical Fund I, L.P. and Vertical Fund II, L.P.;
and

     WHEREAS, the parties hereto desire to amend the Fifth Amended Stockholders'
Agreement on the terms and conditions set forth herein.

     NOW, THEREFORE, in consideration of the premises and of the mutual promises
hereinafter set forth, the parties hereby agree as follows:

     1. DEFINITIONS. Capitalized terms used in this Amendment and not otherwise
defined shall have the meanings ascribed to them in the Fifth Amended
Stockholders' Agreement.

     2. REGISTRATION RIGHTS.

        (a) PIGGYBACK REGISTRATION RIGHTS. The shares of Common Stock acquired
by the Investor pursuant to the Purchase Agreement (the "ADM Shares") shall be
deemed to be "Registrable Securities" under the Fifth Amended Stockholders'
Agreement, entitled to all of the rights and benefits thereunder.

        (b) DEMAND REGISTRATION RIGHTS. The Investor may make one (and only one)
Demand Registration with respect to all or part of the ADM Shares. Such Demand
Registration shall be made pursuant to and subject to all of the terms and
conditions set forth in Article II of

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the Fifth Amended Stockholders' Agreement. Notwithstanding the foregoing, the
following provisions shall supercede the terms and conditions set forth in
Article II of the Fifth Amended Stockholders' Agreement as such provisions apply
to the ADM Shares:

               (i) The Investor may make the demand on its own, without the
          involvement of any other entity, notwithstanding the provisions of
          Section 2.2 that require the holders of at least 30% of the
          Registrable Securities to make such demand;

               (ii) The fourth sentence of Section 2.2 (which contains the
          provisions regarding the 180 day filing delay) shall be deleted and
          replaced with the following sentence:

               "However, the Company will be entitled to postpone, for a
               reasonable period of time, but in no event more than 60 days
               after the date of its receipt of such a request pursuant to this
               Section 2.2, the filing of any registration statement or offering
               and sale, if the Company determines, in its reasonable business
               judgment, that the proposed registration statement or the
               offering would be materially detrimental to the Company and gives
               the Investor written notice of such determination."

          (iii) Notwithstanding the provisions of Section 2.3, the Company shall
     not reduce the number of ADM Shares requested to be registered by the
     Investor pursuant to the exercise of its demand registration right granted
     under Section 2(b) hereof; and

               (iv) Any reference to a "registration statement" in Article II
          shall be deemed to mean a Form S-3 Registration Statement if the
          Company is eligible to use such form. If the Company is not eligible
          to use Form S-3, then such references shall be to a Form S-1
          Registration Statement. In either case, the Company shall keep such
          registration statement effective until the ADM Shares cease to be
          Registrable Securities pursuant to the proviso contained in the first
          sentence of Section 2.4 or pursuant to the last sentence of Section
          2.4 (in each instance, free of the volume limitations of Rule 144 of
          the 1933 Act).

        3. PRE-EMPTIVE RIGHTS WAIVER. The parties hereto acknowledge and agree
that the Investor's purchase of the ADM Shares shall be deemed an "Excluded
Offering" under Article III of the Fifth Amended Stockholders' Agreement and
shall not entitle the Holders to exercise any pre-emptive rights thereunder.

        4. NO OTHER AMENDMENTS. Except as amended by the terms set forth in this
Amendment, all other provisions of the Fifth Amended Stockholders' Agreement
shall remain in full force and effect.

                            [SIGNATURE PAGES FOLLOW.]

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        IN WITNESS WHEREOF, the parties have executed this Amendment as of the
date first set forth above.

                                      METABOLIX INC.

                                      By: /s/ James J. Barber
                                          ---------------------
                                          Name: James J. Barber
                                          Title: President and CEO

                                      INVESTOR:

                                      ARCHER-DANIELS-MIDLAND COMPANY

                                      By: /s/ John D. Rice
                                          ------------------
                                          Name: John D. Rice
                                          Title: Executive Vice President

                                       STOCKHOLDERS:

                                       /s/ Edward M. Giles
                                       --------------------------
                                       Edward M. Giles

                                       STATE FARM MUTUAL
                                       AUTOMOBILE INSURANCE CO.

                                       By: /s/ John Concklin
                                          ------------------
                                          Name: John Concklin
                                          Title: Vice President - Common Stocks

                                      By: /s/ Larry Rottunda
                                          ------------------
                                          Name: Larry Rottunda
                                          Title: Assistant Secretary

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                                            VERTICAL FUND II, L.P.

                                            By:  The Vertical Group, L.P.
                                                 Its General Partner

                                            By: /s/ Jack W. Lasersohn
                                                -----------------------
                                                Name: Jack W. Lasersohn
                                                Title: General Partner

                                            VERTICAL FUND I, L.P.

                                            By:  The Vertical Group, L.P.
                                                 Its General Partner

                                            By: /s/ Jack W. Lasersohn
                                                -----------------------
                                                Name: Jack W. Lasersohn
                                                Title: General Partner<Page>

                                                                   EXHIBIT 10.18

                            STOCK PURCHASE AGREEMENT

     This STOCK PURCHASE AGREEMENT is entered into as of July 12, 2006, by and
among Metabolix, Inc., a Delaware corporation (the "COMPANY"), and
Archer-Daniels-Midland Company, a Delaware corporation ("INVESTOR").

     WHEREAS, the Company has determined to raise $7,500,000 (the "FINANCING")
through the sale of shares of its common stock, par value $.01 per share (the
"COMMON STOCK") to the Investor simultaneously with, and at a price per share
equal to the price per share (the "IPO PRICE") that the Common Stock is being
sold in, the Company's initial public offering ("IPO"); and

     WHEREAS, the Investor, wishes to purchase shares of Common Stock on the
terms and subject to the conditions set forth in this Agreement.

     NOW, THEREFORE, in consideration of the foregoing and the mutual promises
and covenants set forth below, the Investor and the Company hereby agree as
follows:

     1. SALE OF COMMON STOCK.

        (a) PURCHASE AND SALE CLOSING. The Company agrees to issue and sell to
the Investor, and the Investor hereby agrees to purchase from the Company,
$7,500,000 of Common Stock at the IPO Price.

        (b) CLOSING. The closing of the sale and purchase of the shares of
Common Stock (the "CLOSING") shall take place at the offices of Goodwin Procter,
LLP, Exchange Place, Boston, MA 02109, simultaneously with the closing of the
Company's IPO. At the Closing, the Company shall direct its transfer agent to
promptly deliver to the Investor a stock certificate, registered in the name of
such Investor, representing the number of shares of Common Stock being purchased
hereby by such Investor at the Closing. Delivery to the Investor shall be made
against payment to the Company of the full amount of the purchase price for the
shares of Common Stock being purchased by the Investor at the Closing.

        (c) PAYMENT OF PURCHASE PRICE. Payment by the Investor of the purchase
price for the shares of Common Stock shall be made by wire transfer of
immediately available funds to an account specified in writing by the Company.

     2. REPRESENTATIONS AND WARRANTIES.

     2.1 REPRESENTATIONS AND WARRANTIES OF THE COMPANY. Except as set forth on
the disclosure schedules attached hereto (the "DISCLOSURE SCHEDULES") and except
as contemplated in connection with the Company's IPO, the Company hereby
represents and warrants to the Investor as follows:

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        (a) ORGANIZATION AND GOOD STANDING. The Company is a corporation duly
organized, validly existing and in good standing under the laws of the State of
Delaware and has all requisite corporate power and authority to carry on its
business as now conducted and as proposed to be conducted, and is qualified to
do business as a foreign corporation and is in good standing under the laws of
each state or other jurisdiction in which either the ownership or use of the
properties owned or used by it, or the nature of the activities conducted by it,
requires such qualification.

        (b) CAPITALIZATION. The authorized capital stock of the Company and the
outstanding shares of each class and series thereof and of the securities
convertible, exercisable or exchangeable therefor as of the date hereof are as
set forth on SCHEDULE 2.1(b) attached hereto. Except as set forth on SCHEDULE
2.1(b), there are no (i) outstanding warrants, options, agreements, convertible
securities or other commitments or instruments pursuant to which the Company is
or may become obligated to issue or sell any shares of its capital stock or
other securities or (ii) preemptive or similar rights to purchase or otherwise
acquire shares of the capital stock or other securities of the Company (such as
a right of first refusal, right of first offer, proxy, voting trust, voting
agreement, etc.) with respect to the sale or voting of shares of capital or
securities of the Company (whether outstanding or issuable) except to the extent
such rights have been waived.

        (c) AUTHORIZATION. The Company has all requisite corporate power and
authority to execute and deliver this Agreement and any and all instruments
necessary or appropriate in order to effectuate fully the terms and conditions
contained herein and all related transactions and to perform its obligations
hereunder. This Agreement has been duly authorized by all necessary action on
the part of the Company, and has been duly executed and delivered by the Company
and constitutes the valid and legally binding obligation of the Company
enforceable in accordance with its terms and conditions. The authorization,
issuance, sale and delivery of the Common Stock have been duly authorized by all
requisite action of the Company's board of directors and stockholders.

        (d) VALID ISSUANCE OF THE STOCK. When issued in accordance with this
Agreement, the shares of Common Stock, will be (i) duly and validly issued,
fully paid and nonassessable, free of any liens, options, encumbrances, proxies,
adverse claims or restrictions imposed by the Company except as set forth in the
Company's Amended and Restated Certificate of Incorporation, as amended (the
"RESTATED CERTIFICATE OF INCORPORATION"), and (iii) assuming the accuracy of the
Investor's representations in this Agreement at the time of such issuance,
issued in compliance with all applicable federal and state securities laws.

        (e) NON-CONTRAVENTION. No consent, approval, order or authorization of,
or registration, qualification, designation, declaration or filing with, any
federal, state, or local governmental authority (other than filings required to
be made under applicable federal and state securities laws) on the part of the
Company is required in connection with (i) the authorization, execution and
delivery of this Agreement or (ii) the authorization, issuance and delivery of
the Common Stock pursuant to this Agreement. The Company is not in violation or
default of any provisions of the Company's Restated Certificate of Incorporation
or By-Laws, each as amended to date (collectively, the "CHARTER DOCUMENTS") or
of any instrument, judgment, order, writ,

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decree or contract to which the Company is a party or by which the Company is
bound or of any provision of any statute, rule or regulation applicable to the
Company, which violation or default would materially and adversely affect the
business of the Company. The execution, delivery and performance of this
Agreement will not result in any violation of, be in conflict with, or
constitute a default under, with or without the passage of time or the giving of
notice of (w) any provision of the Charter Documents, (x) any provision of any
judgment, decree or order to which the Company is a party or by which the
Company is bound, (y) any material contract, obligation or commitment to which
the Company is a party or by which the Company is bound; or (z) any statute,
rule or regulation applicable to the Company which would materially and
adversely affect the business of the Company.

        (f) COMPLIANCE WITH SECURITIES LAWS. Assuming the accuracy of the
Investors' representations, the offer, grant, sale, and/or issuance of the
Common Stock will not be in violation of the Securities Act of 1933, as amended
(the "1933 ACT"), the Securities Exchange Act of 1934, as amended (the "1934
ACT"), any state securities or "blue sky" law, or the Charter Documents, when
offered, sold and issued in accordance with this Agreement.

     2.2 INVESTOR REPRESENTATIONS

        (a) The Investor represents and warrants to the Company that: (i) it is
an "accredited investor" as defined in Rule 501(a) promulgated under the 1933
Act; (ii) it has sufficient knowledge and experience in investing in companies
similar to the Company in terms of the Company's stage of development, so as to
be able to evaluate the risks and merits of its investment in the Company and it
is able financially to bear the risks thereof; (iii) it has had an opportunity
to discuss the Company's business, management and financial affairs with the
Company's management; and (iv) its financial condition is such that it is able
to bear the risk of holding the Common Stock for an indefinite period of time
and can bear the loss of the entire investment in such securities.

        (b) This Agreement is made in reliance upon the Investor's express
representations that (i) the Common Stock being purchased by such Investor is
being acquired for such Investor's own account (and not on behalf of any other
person or entity) for the purpose of investment and not with a view to, or for
sale in connection with, the distribution thereof, nor with any present
intention of distributing or selling the Common Stock or any portion thereof,
(ii) the Investor was not organized for the specific purpose of acquiring the
Common Stock and (iii) the Common Stock will not be sold by the Investor without
registration under the 1933 Act or applicable state securities laws, or an
exemption therefrom.

        (c) The Investor understands that until such time as the Common Stock
shall have been registered under the 1933 Act and applicable state securities
laws or shall have been transferred in accordance with an opinion of counsel
reasonably satisfactory to the Company that such registration is not required,
stop transfer instructions shall be issued to the Company's transfer agent, if
any, or, if the Company transfers its own securities, a notation shall be made
in the appropriate records of the Company with respect to such securities, and
the certificate or certificates representing such securities shall bear a
restrictive legend stating that such securities

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have not been registered under the 1933 Act and applicable state securities laws
and referring to restrictions on the transferability and sale thereof.

     The Investor further understands that his, her or its representations and
warranties hereunder will not preclude disposition of the Common Stock without
registration thereof, in compliance with Rule 144 promulgated under the 1933 Act
("RULE 144"). The Investor understands and acknowledges, however, that (i) there
is not now available, and may not be available when he, she or it wishes to sell
the Common Stock, or any portion thereof, the adequate current public
information with respect to the Company which would permit offers or sales of
such securities pursuant to Rule 144, and, therefore, compliance with the 1933
Act or some other exemption from the registration and prospectus delivery
requirements of the 1933 Act will be required for any such offer or sale, and
(ii) except as set forth in Amendment No. 1 to the Fifth Amended and Restated
Stockholders' Agreement of even date herewith (the "Amendment to Stockholders'
Agreement"), the Company is under no obligation to register such securities or
to make Rule 144 available.

        (d) RECEIPT OF INFORMATION. The Investor has been furnished access to
the business records of the Company and such additional information and
documents as the Investor has requested and has been afforded an opportunity to
ask questions of and receive answers from representatives of the Company
concerning the terms and conditions of this Agreement, the purchase of the
Common Stock, the Company's business, operations, market potential,
capitalization, financial condition and prospects, and all other matters deemed
relevant by the Investor.

        (e) BROKERAGE. There are no claims for brokerage commissions or finder's
fees or similar compensation in connection with the transactions contemplated by
this Agreement based on any arrangement or agreement made by or on behalf of the
Investor, and the Investor agrees to indemnify and hold the Company harmless
against any damages incurred as a result of any such claims.

        (f) AUTHORIZATION. The Investor has all requisite power and authority to
execute and deliver this Agreement. This Agreement constitutes the valid and
legally binding obligation of the Investor, enforceable against the Investor in
accordance with its terms.

     4. CONDITIONS TO THE CLOSING.

        (a) CONDITIONS TO INVESTORS' OBLIGATION TO PURCHASE THE COMMON STOCK AT
THE CLOSING. The Investor's obligation to purchase the shares of Common Stock to
be purchased by it at the Closing is subject to the satisfaction of the
following conditions:

          (i) REPRESENTATIONS AND WARRANTIES TRUE. The representations and
     warranties made by the Company shall be true and correct as of the Closing
     in all material respects.

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          (ii) CONSENTS, PERMITS, AND WAIVERS. The Company shall have obtained
     any and all consents, permits and waivers necessary or appropriate for
     consummation of the transactions contemplated by this Agreement (except as
     may be properly obtained subsequent to the Closing).

          (iii) INITIAL PUBLIC OFFERING. The Company shall be simultaneously
     closing its IPO.

          (iv) AMENDMENT TO STOCKHOLDERS' AGREEMENT. The Amendment to
     Stockholders' Agreement shall be in full force and effect.

        (b) CONDITIONS TO OBLIGATIONS OF THE COMPANY AT THE CLOSING. The
Company's obligation to issue and sell shares of Common Stock at the Closing is
subject to the satisfaction of the following conditions:

          (i) REPRESENTATIONS AND WARRANTIES TRUE. The representations and
     warranties made by the Investor shall be true and correct as of the Closing
     in all material respects.

          (ii) CONSENTS, PERMITS, AND WAIVERS. The Company shall have obtained
     any and all consents, permits and waivers necessary or appropriate for
     consummation of the transactions contemplated by this Agreement (except as
     may be properly obtained subsequent to the Closing).

          (iii) INITIAL PUBLIC OFFERING. The Company shall be simultaneously
     closing its IPO.

     5. NOTICES. Notices given hereunder shall be deemed to have been duly given
on (i) the date of personal delivery, or (ii) on the date one day after
delivered to a reputable overnight courier with proper delivery instructions, to
the party being notified at his, her, or its address specified on the applicable
signature page hereto or such other address as the addressee may subsequently
notify the other party of in writing.

     6. ENTIRE AGREEMENT AND AMENDMENTS. This Agreement constitutes the entire
agreement of the parties with respect to the subject matter hereof. Neither this
Agreement nor any provision hereof may be waived, modified, amended or
terminated except by a written agreement signed by the Company and the Investor.
To the extent any other indenture, agreement or instrument by which any party
hereto is bound conflicts with this Agreement, this Agreement shall have
precedence over such conflicting term or provision.

     7. GOVERNING LAW; SUCCESSORS AND ASSIGNS. This Agreement shall be governed
by the laws of the Commonwealth of Massachusetts and shall be binding upon the
heirs, personal representatives, executors, administrators, successors and
assigns of the parties without regard to the conflict of laws principles
therein.

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     8. WAIVERS. No waiver of any breach or default hereunder shall be
considered valid unless in writing, and no such waiver shall be deemed a waiver
of any subsequent breach or default of the same or similar nature.

     9. SEVERABILITY. The invalidity or unenforceability of any particular
provision of this Agreement shall not affect the other provisions hereof, and
this Agreement shall be construed in all respects as if such invalid or
unenforceable provision were omitted, provided that such construction shall not
substantially impair the bargained-for rights of any party hereto.

     10. BENEFIT AND BURDEN; ASSIGNS. This Agreement may not be assigned by
either party without the consent of the other party.

     11. CAPTIONS. Captions are for convenience only and are not deemed to be
part of this Agreement.

     12. COUNTERPARTS. This Agreement may be executed in counterparts, each of
which shall be deemed an original, but all of which together shall constitute
one and the same instrument.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

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     IN WITNESS WHEREOF, this Stock Purchase Agreement has been executed as of
the date and year first above written.

                                     COMPANY:

                                     METABOLIX, INC.

                                     By: /s/ James Barber
                                         ------------------------------
                                     Name:  Dr. James Barber
                                     Title: President

                                     Address: 21 Erie Street
                                              Cambridge, MA 02139-4260

                                     INVESTOR:

                                     ARCHER-DANIELS-MIDLAND COMPANY

                                     By: /s/ John D. Rice
                                         ------------------------------
                                     Name:   John D. Rice
                                     Title:  Executive Vice President

                                     Address: 4666 Faries Parkway
                                              Decatur, Illinois 62526

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