Document:

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                                                                     EXHIBIT 4.2

                  AMENDMENT NO. 1 TO LETTER AGREEMENT REGARDING
                      PARTICIPATION IN CARRIZO 2001 PROGRAM

      Carrizo Oil & Gas, Inc., a Texas corporation ("Carrizo"), Berea
Associates, LLC, Berea Oil & Gas Corp., PAC Finance (USA) Inc., William R.
Ziegler, Thomas H. O'Neill, Jr. and Berea Associates II LLC, entered into a
Letter Agreement Regarding Participation in Carrizo 2001 Program dated as of May
1, 2001 (the "Letter Agreement"), and by this agreement hereby, pursuant to
Section 12 of the Letter Agreement, amend the Letter Agreement effective as of
June 1, 2001 as follows (all capitalized terms used herein which are defined
in the Letter Agreement shall have the meanings assigned to them in the Letter
Agreement unless otherwise defined herein):

                                   ARTICLE I.

      Subsection 8(c) of the Letter Agreement shall be amended and restated in
its entirety as follows:

            (c) REGISTRATION OF COMMON STOCK. If Carrizo has in effect a shelf
      registration statement under the Securities Act of 1933 than can be used
      in compliance with the laws, rules, regulations and interpretations of the
      Securities and Exchange Commission (the "SEC") (the parties recognizing
      that such use would not currently be in compliance with the laws, rules,
      regulations and interpretations of the SEC) to register the issuance of
      the Common Stock provided for herein, then the Common Stock shall be
      issued pursuant to such registration statement. If the Common Stock cannot
      be issued pursuant to an effective registration statement, and the Common
      Stock is therefore restricted as provided below, then the exchanging
      Participants shall have piggy-back registration rights for sales by other
      Carrizo shareholders on customary terms which shall be consistent with and
      subject to the restrictions, limitations and covenants of (including,
      without limitation, by including counterparts of Sections 5.9, 5.12, 5.13
      and 6.4 of) the Registration Rights Agreement dated as of December 15,
      1999 by and among Carrizo, CB Capital Investors, L.P. and Mellon Ventures,
      L.P. and the Amended and Restated Registration Rights Agreement dated
      December 15, 1999 by and among Carrizo, Douglas A. P. Hamilton, Paul B.
      Loyd, Jr., Steven A. Webster, Frank A. Wojtek, S. P. Johnson IV and DAPHAM
      Partnership, L.P., but such piggy-back rights shall in all events be
      subject to the limitations imposed by Section 5.5 of both such agreements.

                                  ARTICLE II.

      Subsection 8(d) of the Letter Agreement shall be amended by inserting
the following sentence after the last sentence of the paragraph:

            The Participants agree to keep strictly confidential all nonpublic
      information received in connection with the preceding sentence.

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<PAGE>   2

                                  ARTICLE III.

      Subsection 8(e) of the Letter Agreement shall be amended by inserting the
following sentence at the end of the Subsection:

            The Participants agree to comply with the restrictions contemplated
      by such legend and further agree that any offer, sale, assignment,
      transfer, pledge or disposition of an interest in the Program or the
      option provided in Section 8(a) hereof shall be subject to the same
      restrictions on transfer as are applicable to the Common Stock.

                                  ARTICLE IV.

      A new subsection 8(g) will be added to the Letter Agreement to read in its
entirety as follows:

            (g) ADJUSTMENT FOR STOCK SPLITS AND REVERSE STOCK SPLITS. If at any
      time after the date of this Agreement, Carrizo shall subdivide, split or
      reclassify its outstanding shares of Common Stock into a larger number of
      shares of Common Stock, or combine its outstanding shares of Common Stock
      into a smaller number of shares of Common Stock, then the number of shares
      of Common Stock issuable upon the exercise of the Participant's option to
      convert its interest in the Program to Common Stock shall be adjusted so
      as to equal the number of shares of Common Stock (or other securities)
      that such Participant would have held immediately after the occurrence of
      such event if the Participant has exercised its option immediately prior
      to such event.

                                   ARTICLE V.

      A new subsection 8(h) will be added to the Letter Agreement to read in its
entirety as follows:

            (h) CONSOLIDATION, MERGER OR SALE OF SUBSTANTIALLY ALL OF CARRIZO'S
      ASSETS. If at any time after the date of this Agreement, there shall be
      any consolidation, merger, conversion or other business combination of
      Carrizo with another entity or any sale or conveyance by Carrizo of all or
      substantially all of its assets or property to another entity, and in any
      such case the Common Stock is to be exchanged or converted into shares of
      stock, other securities, cash or other property, then in each case the
      exchange option provided for herein shall thereafter be exercisable for
      the kind and number of shares of stock, other securities, cash or other
      property to which a holder of the number of shares of Common Stock
      issuable upon the exercise of such exchange option would have been
      entitled upon such event.

                                  ARTICLE VI.

      This Agreement is effective as of the date first written above. In witness
of the foregoing, Carrizo and the undersigned Participants hereby execute this
Agreement as of the date first written above.

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<PAGE>   3

                                          Sincerely,

                                          CARRIZO OIL & GAS, INC.

                                          By:
                                             -----------------------------------
                                          Name:
                                               ---------------------------------
                                          Title:
                                                --------------------------------

ACCEPTED AND AGREED TO BY
  THE FOLLOWING PARTICIPANTS:

Berea Associates, LLC

By:
   -----------------------------------
Name:
     ---------------------------------
Title:
      --------------------------------

Berea Oil & Gas Corp.

By:
   -----------------------------------
Name:
     ---------------------------------
Title:
      --------------------------------

PAC Finance (USA) Inc.

By:
   -----------------------------------
Name:
     ---------------------------------
Title:
      --------------------------------

--------------------------------------
William R. Ziegler

--------------------------------------
Thomas H. O'Neill, Jr.

                                       3
<PAGE>   4

Berea Associates II LLC
by Berea Oil & Gas Corp.
its Managing Member

By:
   -----------------------------------
    Thomas H. O'Neill, Jr.

                                       4<PAGE>   1
                                                                     EXHIBIT 4.3

                                 PROMISSORY NOTE

U.S.$7,500,000.00                                                  June 30, 2001

                  FOR VALUE RECEIVED, CCBM, Inc. (together with its successors
and assigns, the "Maker") by this promissory note (this "Note") hereby
unconditionally promises to pay to the order of ROCKY MOUNTAIN GAS, INC.
(together with its successors and assigns, "Payee"), the principal sum of Seven
Million Five Hundred Thousand and No/One-Hundredths Dollars (U.S.$7,500,000.00)
in installments as hereinafter provided and to pay interest on the principal
balance hereof from time to time outstanding, as hereinafter provided, at the
rate of 8% per annum.

                  The terms used in this Note shall have the following meanings:

                  (a) The term "Business Day" shall mean any day upon which
commercial banks are open for the transaction of business in Houston.

                  (b) The term "Mortgaged Properties" shall have the meaning
given to such term in the Mortgage and Financing Statement dated June 9, 2001,
as the same may be modified or amended from time to time.

                  (c) The term "Loan Documents" shall collectively mean this
Note, the Mortgage and Financing Statement and all other documents and
instruments now or hereafter executed and delivered in connection therewith or
otherwise pertaining to this Note, as the same may be modified or amended from
time to time.

                  The principal hereof shall be paid in forty-one (41)
installments, of $125,000 each, payable on the last Business Day of each month,
commencing July 31, 2001 (each a "Payment Date"). On December 31, 2004, the
Maker shall repay in full the outstanding principal, together with accrued but
unpaid interest.

                  Interest on this Note is payable on each Payment Date,
beginning on July 31, 2001. Interest will be calculated on the basis of the
actual number of days elapsed (including the first day, but excluding the last
day) over a year of 365 days.

                  This Note may be prepaid, in whole or in part, at any time
without penalty or premium.

                  The Maker hereby waives demand, diligence, presentment,
protest and notice of every kind, and warrants to the holder that all action and
approvals required for the execution and delivery hereof as a legal, valid and
binding obligation of the undersigned, enforceable in accordance with the terms
hereof, have been duly taken and obtained.

                  This Note is secured by certain leasehold interests and
property rights which are identified in the Mortgage and Financing Statement,
together with all of Maker's right, title and interest therein or otherwise
described as set forth therein.
<PAGE>   2

                  Payee shall not enforce the liability and obligation of Maker
under this Note and the other Loan Documents by any action or proceeding wherein
a money judgment shall be sought against Maker, except that Payee may bring a
foreclosure action or any other appropriate action or proceeding to enable Payee
to enforce and realize upon its interest in the Mortgaged Properties; provided
any judgment in any such action or proceeding shall be enforceable against Maker
only to the extent of Maker's interest in the Mortgaged Properties, and Payee,
by accepting this Note, agrees that it shall not sue for, seek or demand any
deficiency judgment against Maker in any such action or proceeding under or by
reason of or under or in connection with this Note or any other Loan Document.

                  In the event Maker defaults on this Note and Payee is required
to bring legal action against Maker on this Note, Maker shall be entitled to
recover all attorney's fees, costs and expenses incurred as a result of such
default.

                  THIS NOTE SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE
WITH, THE LAW OF THE STATE OF WYOMING, U.S.A.

                                   CCBM, Inc.

                                   By:
                                        ----------------------------------------
                                                     (Signature)

                                   Name:
                                          --------------------------------------
                                                       (Print)

                                   Title:
                                           -------------------------------------

ACCEPTED:

ROCKY MOUNTAIN GAS, INC.

By:
     -------------------------------

Name:
       -----------------------------

Title:
        ----------------------------

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