Exhibit 4.03


                                  April , 2002

Neidiger, Tucker, Bruner, Inc.
1675 Larimer Street, Suite 300
Denver, Colorado  80202

         Re:      Natural Gas Services Group, Inc.

Ladies and Gentlemen:

         Reference is made to a proposed public offering of shares of common
stock and warrants (the "Public Securities") by Natural Gas Services Group, Inc.
(the "Company") pursuant to a Registration Statement (the "Registration
Statement") and prospectus included therein to be filed with the Securities and
Exchange Commission and to be underwritten by Neidiger/Tucker/Bruner, Inc. as
the managing underwriter (the "Underwriter") named in an underwriting agreement.

         In consideration of the offer and sale of the Public Securities by the
Company and the Underwriter and of other valuable consideration, the receipt of
which is hereby acknowledged, the undersigned agrees not to offer, sell
(including engaging in a short sale), contract to sell, pledge, hypothecate,
grant any option to purchase or otherwise dispose of any shares of common stock
of the Company or any securities convertible into or exchangeable for common
stock of the Company beneficially owned or otherwise held by the undersigned as
of the date of this letter or acquired on or prior to the date of effectiveness
of the Registration Statement or issuable upon exercise of options or warrants
held by the undersigned on such dates (collectively, the "Shares") for the
period specified hereafter without the prior written consent of the Underwriter.
Such restrictions shall apply to the total number of Shares held by the
undersigned for a period of one year after the date of the effective date of the
Registration Statement.

         As a reasonable means of ensuring compliance with the terms of this
Agreement, the undersigned further agrees that the Company may instruct the
transfer agent for the Shares to place a transfer restriction on such transfer
agent's records.

         Notwithstanding the foregoing, if the undersigned is an individual, he
or she may transfer any or all of the Shares either during his or her lifetime
or on death by will or intestacy to his or her immediate family or to a trust,
the beneficiaries of which are exclusively the undersigned and/or a member or
members of his or her immediate family; provided, however, that in any such case
it shall be a condition to the transfer that the transferee execute an agreement
stating that the transferee is receiving and holding the Shares subject to the
provisions of this Agreement, and there shall be no further transfer of such
Shares except in accordance with this



Neidiger/Tucker/Bruner, Inc.
Agreement Not To Sell
Page 2


Agreement. For purposes of this paragraph, "immediate family" shall mean spouse,
lineal descendant, father, mother, brother or sister of the transferor.

         In addition, notwithstanding the foregoing, if the undersigned is a
partnership, the partnership may transfer any Shares to a partner of such
partnership or a retired partner of such partnership who retires after the date
hereof, or to the estate of any such partner or retired partner, and any partner
who is an individual may transfer Shares by gift, will or intestate succession
to his or her immediate family (as defined above) or ancestors; and if the
undersigned is a corporation, the corporation may transfer Shares to any
shareholder of such corporation and any shareholder who is an individual may
transfer Shares by gift, will or intestate succession to his or her immediate
family (as defined above) or ancestors; provided, however, that in any such
case, it shall be a condition to the transfer that the transferee execute an
agreement stating that the transferee is receiving and holding the Shares
subject to the provisions of this Agreement, and there shall be no further
transfer of such Shares except in accordance with this Agreement.

         Finally, notwithstanding the foregoing, the Underwriter, in its sole
discretion, may release from this Agreement any or all of the Shares or release
from any agreement similar to this Agreement any shares of common stock owned by
any other shareholder of the Company. Any such release must be in a writing
signed by the Underwriter.

         This agreement shall be enforceable by the Company and the Underwriter,
or either of them, and shall bind and inure to the benefit of their respective
successors, personal representatives, heirs, and assigns.


                                           Very truly yours,


                                           By:
_______________________________                ________________________________
Shares of common stock subject                 Signature
to this Agreement


                                           _____________________________________
                                             Printed name of person or entity

                                           _____________________________________
                                               Title if signing for an entity