Exhibit 10.12.1    December 17,
2008  Joseph
Hill c/o Metabolix, Inc. 21 Erie Street Cambridge, MA 02139  Re:  First Amendment to Employment Agreement  Dear
Joe:  This letter amends the terms of the Employment Letter
Agreement (the Agreement) dated as of March 14, 2008, by and between
Metabolix, Inc. (the Company) and you as set forth below.  Capitalized terms not defined herein shall
have the meaning specified in the Agreement.  1.Section 3(a) of
the Agreement is hereby amended by inserting the following after the word Company
and prior to the word will in the second sentence thereof:  will pay such base
salary on a semi-monthly basis in accordance with the Companys normal payroll
practices and  2.Section 4(f) of
the Agreement is hereby amended by deleting said section in its entirety and
substituting the following therefor:  (f)Definition
of Good Reason.  As used in this
Agreement, Good Reason means that you have complied with the Good
Reason Process (hereinafter defined) following the occurrence of any of the
following events:  (i) a material
diminution in your responsibilities, authority or duties; (ii) a material
diminution in your Base Salary; (iii) a material change in the geographic
location at which you provide services to the Company; or (iv) the
material breach of this Agreement by the Company.  Good Reason Process shall mean that (i) you
reasonably determine in good faith that a Good Reason condition has occurred;
(ii) you notify the Company in writing of the occurrence of the Good
Reason condition within 60 days of the occurrence of such condition; (iii) you
cooperate in good faith with the Companys efforts, for a period not less than
30 days following such notice (the Cure Period), to remedy the
condition; (iv) notwithstanding such efforts, the Good Reason condition
continues to exist; and (v) you terminate your employment within 60 days
after the end of the Cure Period.  If the
Company cures the Good Reason condition  Metabolix  |  21 Erie Street  | 
Cambridge  |  MA 
|  02139  |  USA tel: 617 583 1700  |  fax: 617 583 1767  | 
www.metabolix.com


during the Cure Period,
Good Reason shall be deemed not to have occurred.  3.Section 5(a) of
the Agreement is hereby amended by adding the following immediately before the
period at the end of the last sentence thereof:  but in no event more
than 45 days after the date on which your employment terminates  4.Sections
5(b) and 5(c) of the Agreement are each hereby amended by inserting
the following immediately at the end of the parenthetical and prior to the
comma within the first sentence thereof:  within thirty (30) days
after the date of termination  5.Sections
5(b) and 5(c) of the Agreement are each hereby amended by deleting
the upon your delivery of the Release at the end thereof and substituting
therefor:  , commencing on the 37th day after the date on which your employment
terminates (provided the Release is effective prior to such date) payable in
accordance with the Companys normal payroll practices, provided that the first
payment will include all amounts which would have been paid in the 37 days
following your termination of employment.  6.Section 5(d) of
the Agreement is hereby amended by deleting the first and second sentences
thereof.  7.Section 5(e) of
the Agreement is hereby amended (x) by deleting the second sentence
thereof and (y) by deleting , then following the word Agreement and
prior to the term in addition within the first sentence thereof and
substituting the following therefor:  it shall be considered a
termination by the Company without Cause and  8.Section 5(g) of
the Agreement is hereby amended by deleting the word you after the term pay
to and prior to the term an additional amount within the second sentence
thereof, and substituting the following therefor:  the relevant taxing
authority as withholding taxes at such time or times when each payment of
Excise Tax is due,  9.Section 5(i) of
the Agreement is hereby amended to provide that the term separation from
service shall be defined in accordance with Section 409A(a)(2)(A)(i) of
the Code and the applicable provisions of Treasury Regulation Section 1.409A-1(h).  2


10.The
Agreement is hereby amended by inserting the following as a new Section 5(j) to
the Agreement:  (j)Section 409A.  (i) Anything in this Agreement to the
contrary notwithstanding, if at the time of your separation from service, the
Company determines that the you are a specified employee within the meaning
of Section 409A(a)(2)(B)(i) of the Code, then to the extent any
payment or benefit that you become entitled to under this Agreement on account
of your separation from service would be considered deferred compensation
subject to the 20 percent additional tax imposed pursuant to Section 409A(a) of
the Code as a result of the application of Section 409A(a)(2)(B)(i) of
the Code, such payment shall not be payable and such benefit shall not be
provided until the date that is the earlier of (A) six months and one day
after your separation from service, or (B) your death.  If any such delayed cash payment is otherwise
payable on an installment basis, the first payment shall include a catch-up
payment covering amounts that would otherwise have been paid during the
six-month period but for the application of this provision, and the balance of
the installments shall be payable in accordance with their original schedule.  (ii)Solely
for purposes of Section 409A of the Code, each installment payment
described in Section 5 is considered a separate payment.  11.Section 9(e) of
the Agreement is hereby amended by deleting the first sentence of such section
and substituting the following therefore:  The Company shall cause
its rights and obligations hereunder to be assumed by any person or entity that
succeeds to all or substantially all of the Companys business or that aspect
of the Companys business in which you are principally involved and may assign
its rights and obligations hereunder to any Company Affiliate.  12.Section 9(k) of
the Agreement is hereby amended by inserting the following after the word Agreement
and prior to the word comply within the second sentence thereof:  be exempt from, or  13.Except
as amended herein, the Agreement is hereby confirmed in all other respects and
nothing contained herein shall be deemed a waiver of any right or abrogation of
any obligation otherwise existing under the Agreement except to the extent
specifically provided for herein.  3


Please indicate your acceptance of this amendment to
the Agreement by signing the enclosed copy of this letter and returning it to
me.   METABOLIX, INC.By:
  /s/ Richard P. EnoRichard P. EnoPresident & CEOAccepted and agreed:/s/ Joseph D. HillJoseph D. HillDate:
  12-23-08  4