EXHIBIT
4.A.1














Colorado
Interstate Gas Company








and




Harris
Trust and Savings Bank, Trustee








___________








FIRST
SUPPLEMENTAL INDENTURE


Dated
as of June 27, 1997




to




INDENTURE


Dated
as of June 27, 1997











___________











$100,000,000




6.85%
Senior Debentures Due June 15, 2037




















FIRST SUPPLEMENTAL
INDENTURE, dated as of June 27, 1997, between Colorado Interstate Gas Company, a
Delaware corporation (the “Company”), and Harris Trust and Savings Bank, a
banking corporation organized under the laws of the State of Illinois, as
Trustee (the “Trustee”), to the Indenture dated as of June 27, 1997 (the
“Indenture”), between the Company and the Trustee.




Pursuant to Section
9.01(6) of the Indenture, each party agrees as follows for the benefit of the
other party and for the equal and ratable benefit of the Holders of the
Company’s Debentures (as defined below) to supplement the Indenture to establish
the form and terms of a Series of Securities of the Company under the Indenture
to be entitled the “6.85% Senior Debentures Due June 15, 2037”:






ARTICLE
ONE




FORM
AND TERMS OF SECURITIES




SECTION
1.01  Terms of Securities To Be
Issued.




(a)  A Series of
Securities which shall be designated the “6.85% Senior Debentures Due June 15,
2037” (the “Debentures”) shall be executed, authenticated and delivered in
accordance with the provisions of, and shall in all respects be subject to, the
definitions, terms, conditions and covenants of the Indenture, including without
limitation the terms set forth in this First Supplemental
Indenture.




(b)  Pursuant to Section
2.02 of the Indenture, the Debentures shall initially be issued in the form of
one or more Global Securities.




(c)  The aggregate
principal amount of Debentures which maybe
authenticated and delivered under this First Supplemental Indenture shall not
exceed $100,000,000 (except for Debentures authenticated and delivered upon
transfer of, or in exchange for, or in lieu of, other Debentures pursuant to
Section 2.07, 2.08, 2.11, 3A.08 or 9.05 of the Indenture).  The entire
amount of Debentures may forthwith be executed by the Company and delivered to
the Trustee and shall be authenticated by the Trustee and delivered to or upon
the order of the Company pursuant to Section 2.03 of the Indenture.




(d)  The Debentures
shall mature on June 15, 2037.




(e)  The Debentures
shall bear interest, and such interest shall be payable, as provided in Article
Two hereof.




SECTION
1.02  Form of the Debentures.




The Debentures and
the Trustee’s certificate of authentication shall be substantially in the form
of Exhibit A annexed hereto.  The terms and provisions contained in
the Debentures shall constitute and are hereby expressly made a part of this
First Supplemental Indenture.









ARTICLE
TWO




PAYMENT
OF INTEREST ON THE DEBENTURES




SECTION
2.01  Interest Payment
Dates.




(a)  The Debentures
shall bear interest from June 27, 1997 or from the most recent Interest Payment
Date (as hereinafter defined) to which interest has been paid or duly provided
for, at the rate of 6.85% per annum, and such interest shall be payable each
June 15 and December 15 (each an “Interest Payment Date”) to the person in whose
name the Debenture is registered at the close of business on the regular record
date (the “Regular Record Date”) for any such Interest Payment Date, which shall
be the last day of the month preceding the month in which such Interest Payment
Date occurs (whether or not a Business Day).




(b)  The first Interest
Payment Date on the Debentures shall be December 15,1997.






ARTICLE
THREE




REDEMPTION




SECTION
3.01  Redemption at the Option of the
Company.




The Debentures are
not redeemable at the option of the Company prior to their
maturity.




SECTION
3.02  Redemption at the Option of the
Holders.




(a)  On June 15, 2007,
or if such date is not a Business Day, then the next succeeding Business Day
(the “Redemption Date”), each Holder of Debentures will have the right (the
“Redemption Right”) to require the Company to redeem all or any part (equal to
$1,000 in principal amount or an integral multiple thereof) of such Holder’s
Debentures, in accordance with this Section 3.02, at a redemption price equal to
100% of the aggregate principal amount thereof (the “Redemption Price”) plus
accrued and unpaid interest, if any, to the Redemption Date.




(b)  On or prior to
March 30, 2007, the Company shall send, by first class mail, postage prepaid, a
notice of the Redemption Right to each Holder of Debentures at its address
appearing in the Security Register, with a copy to the Trustee, which notice
shall contain all instructions and materials necessary to enable such Holder to
submit its Debentures for redemption pursuant to the Redemption
Right.  Such notice shall state:







(1)  that such Holder
has the Redemption Right pursuant to this Section 3.02;




(2)  the Redemption
Date;




(3)  the Redemption
Price (including the amount of any accrued and unpaid interest);




(4)  that any Debenture
not submitted for redemption will continue to accrue interest;




(5)  that on the
Redemption Date the Redemption Price shall become due and payable upon each
Debenture submitted for redemption pursuant to the Redemption Right and that,
unless the Company defaults in paying the Redemption Price therefor, any
Debenture submitted for redemption pursuant to the Redemption Right shall cease
to accrue interest after the Redemption Date;




(6)  that any Holder
electing to have a Debenture redeemed pursuant to the Redemption Right will be
required to surrender the Debenture, or transfer the Debenture by book-entry,
with the form entitled “Option of Holder to Elect Redemption on June 15, 2007”
on the reverse side of the Debenture completed, to the Trustee during the period
from April 15, 2007 and prior to 5:00 p.m.  (New York City time) on
May 15, 2007 (or if such date is not a Business Day, then the next succeeding
Business Day) at the address specified in such notice;




(7)  that any election
on the part of a Holder to exercise the Redemption Right in accordance with this
Section 3.02 shall be irrevocable on the part of the Holder and may not be
withdrawn; and




(8)  that Holders whose
Debentures are being redeemed only in part will be issued a new Debenture or
Debentures equal in principal amount to the unredeemed portion of the Debentures
surrendered (which unredeemed portion must be equal to $1,000 in principal
amount or an integral multiple thereof).




(c)  On or prior to the
Redemption Date, the Company shall deposit with the Trustee or with the Paying
Agent (or, if the Company is acting as its own Paying Agent, segregate and hold
in trust as provided in Section 2.05 of the Indenture) an amount of money
sufficient to pay the Redemption Price of, and (except if the Redemption Date
shall be an Interest Payment Date) any accrued and unpaid interest on, all
Debentures or portions thereof which are to be redeemed on that
date.




(d)  The Debentures to
be redeemed shall, on the Redemption Date, become due and payable at the
Redemption Price thereof and from and after such date (unless the Company shall
default in the payment of the Redemption Price) such Debentures shall cease to
bear interest.  Upon submission of any Debenture for redemption in
accordance with this Section 3.02, such Debenture shall be paid by the Company
at the Redemption Price, plus accrued and unpaid interest, if any, to the
Redemption Date; provided, however,that installments of interest whose Stated Maturity is on or prior to the
Redemption Date shall be payable to the Holders of such Debentures registered as
such on the relevant Regular or Special Record Date according to the terms and
provisions of such Debentures and Section 2.13 of the Indenture.







(e)  If any Debenture is
to be redeemed in part, the Company shall execute and the Trustee shall
authenticate and deliver to the Holder of such Debenture, without service
charge, a new Debenture or Debentures equal in principal amount to the
unredeemed portion of the Debenture surrendered (which unredeemed portion must
be equal to $1,000 in principal amount or an integral multiple
thereof).




(f)  The Company shall
comply with the requirements of Rule 14e-1 under the Securities Exchange Act of
1934, as amended, and any other securities laws and regulations thereunder to
the extent such laws and regulations are applicable in connection with the
redemption of Debentures pursuant to the Redemption Right.  To the
extent the provisions of any such rule conflict with the provisions of the
Indenture relating to the Redemption Right, the Company shall comply with the
provisions of such rule and be deemed not to have breached its obligations
relating to the Redemption Right by virtue thereof.






ARTICLE
FOUR




MISCELLANEOUS




SECTION
4.01  Definitions.




All the terms used
in this First Supplemental Indenture which are defined in the Indenture shall
have the meanings specified in the Indenture, unless otherwise provided herein
or unless the context of this First Supplemental Indenture otherwise
requires.




SECTION
4.02  Governing Law.




THE LAWS OF THE
STATE OF NEW YORK SHALL GOVERN THIS FIRST SUPPLEMENTAL INDENTURE AND THE
DEBENTURES WITHOUT REGARD TO PRINCIPLES OF CONFLICTS OF LAW.




SECTION
4.03  Successors.




All agreements of
the Company in this First Supplemental Indenture and the Debentures shall bind
its successor.  All agreements of the Trustee in this First
Supplemental Indenture shall bind its successor.




SECTION
4.04  Duplicate
Originals.




The parties may
sign any number of copies of this First Supplemental Indenture.  Each
signed copy shall be an original, but all of them together represent the same
agreement.







SIGNATURES




IN WITNESS WHEREOF,
the parties hereto have caused this First Supplemental Indenture to be duly
executed, and their respective corporate seals to be hereunto affixed and
attested, all as of the date first written above.




COLORADO INTERSTATE GAS
  COMPANY[SEAL]Attest:
              /s/ Austin
      M.O’Toole
              
              By:
              /s/ Donald W.
      GullquestName: 
      Austin M. O’Toole
                Title: 
      Senior Vice President
                and
      Secretary
              
              
              Name: 
      Donald H. Gullquist
                Title: 
      Senior Vice PresidentHARRIS TRUST AND SAVINGS
  BANK[SEAL]Attest:
              /s/ Amy
      Robert
              
              By:
              /s/Therese
      GaballahTherese
      Gaballuh
                Trust
Officer

















EXHIBIT
A




[FORM OF FACE OF
DEBENTURE]




[APPLICABLE TO
GLOBAL DEBENTURE:]




[UNLESS THIS
CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITORY TRUST
COMPANY (“DTC”) (55 WATER STREET, NEW YORK, NEW YORK) TO THE ISSUER OR ITS AGENT
FOR REGISTRATION OF TRANSFER, EXCHANGE OR PAYMENT AND ANY CERTIFICATE ISSUED IS
REGISTERED IN THE NAME OF CEDE & CO.  OR SUCH OTHER NAME AS IS
REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC AND ANY PAYMENT IS MADE TO CEDE
& CO., OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED
REPRESENTATIVE OF DTC, ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE OR
OTHERWISE BY OR TO ANY PERSON IS WRONGFUL INASMUCH AS THE REGISTERED OWNER
HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.  UNLESS IT IS
EXCHANGED IN WHOLE OR IN PART FOR THE INDIVIDUAL DEBENTURES REPRESENTED HEREBY,
THIS GLOBAL DEBENTURE MAY NOT BE TRANSFERRED EXCEPT AS A WHOLE BY DTC TO A
NOMINEE OF DTC OR BY A NOMINEE OF DTC TO DTC OR ANOTHER NOMINEE OF DTC OR BY DTC
OR ANY SUCH NOMINEE TO A SUCCESSOR OF DTC OR A NOMINEE OF SUCH
SUCCESSOR.]




COLORADO INTERSTATE
GAS COMPANY




6.85% SENIOR
DEBENTURES




DUE JUNE 15,
2037




Rate of Interest
            Maturity Date
            Original Issue Date6.85%
            June 15,
      2037
            June 27,
      1997No.
            
            $





Colorado Interstate
Gas Company, a corporation duly organized and existing under the laws of the
State of Delaware (herein called the “Company”), for value received, hereby
promises to pay to _____________________ or registered assigns, the principal
sum of __________________ on the Maturity Date shown above, and to pay interest
thereon, at the annual rate of interest shown above, from the Original Issue
Date shown above or from the most recent Interest Payment Date (as hereinafter
defined) to which interest has been paid or duly provided for, payable
semi-annually on June 15 and December 15 of each year and at maturity (an
“Interest Payment Date”), commencing on the first such date after the Original
Issue Date shown above.  Interest will be computed on the basis of a
360-day year of twelve 30-day months.




The interest so
payable, and punctually paid or duly provided for, on any Interest Payment Date
will, as provided in the Indenture, be paid to the person in whose name this
Debenture is registered at the close of business on the Regular Record Date for
any such Interest Payment Date, which shall be the last day of the month
preceding the month in which such Interest Payment Date occurs (whether or not a
Business Day).  Any such interest not so punctually paid or duly
provided for, and any interest payable on such defaulted interest (to the extent
lawful), will forthwith cease to be payable to the Holder on such Regular Record
Date and shall be paid to the person in whose name this Debenture is registered
at the close of business on a special record date for the payment of such
defaulted interest to be fixed by the Company, notice of which shall be given to
Holders of Debentures not less than 15 days prior to such special record
date.  Payment of the principal of and interest on this Debenture will
be made at the agency of the Company maintained for that purpose in New York,
New York and at any other office or agency maintained by the Company for such
purpose, in such coin or currency of the United States of America as at the time
of payment is legal tender for payment of public and private debts; provided, however, that, at
the option of the Company, payment of interest, other than interest due on the
Maturity Date, may be made by check mailed to the address of the person entitled
thereto as such address shall appear in the Security Register.




Reference is hereby
made to the further provisions of this Debenture set forth on the reverse
hereof, which further provisions shall for all purposes have the same effect as
if set forth at this place.




Unless the
certificate of authentication hereon has been executed by the Trustee referred
to on the reverse hereof by manual signature, this Debenture shall not be
entitled to any benefit under the Indenture or any supplemental indenture
thereto or be valid or obligatory for any purpose.




A-


IN WITNESS WHEREOF,
COLORADO INTERSTATE GAS COMPANY has caused this instrument to be executed in its
corporate name by the facsimile signature of its duly authorized officers and
has caused a facsimile of its corporate seal to be affixed hereunto or imprinted
hereon.




COLORADO
INTERSTATE GAS


COMPANY











By:


Name:




Title:




ATTEST:











By:


Name:


Title:





DATED:





TRUSTEE’S
CERTIFICATE OF AUTHENTICATION




This is one of the
6.85% Debentures Due June 15, 2037 referred to in the within-mentioned Indenture
and is the Series designated in the within-mentioned First Supplemental
Indenture.




HARRIS
TRUST AND SAVINGS


BANK,
as Trustee











By:


Authorized Signatory















A-


[FORM OF REVERSE
SIDE]




COLORADO INTERSTATE
GAS COMPANY




6.85% SENIOR
DEBENTURES




DUE JUNE15, 2037




This Debenture is
one of a duly authorized issue of Debentures of the Company (which term includes
any successor corporation under the Indenture herein referred to) designated as
its 6.85% Senior Debentures Due June 15, 2037 (the “Debentures”), issued or to
be issued pursuant to an Indenture, dated as of June 27, 1997 (the “Indenture”),
between the Company and Harris Trust and Savings Bank, as Trustee (the
“Trustee”, which term includes any successor trustee under the Indenture), and
under the First Supplemental Indenture, dated as of June 27, 1997 (the “First
Supplemental Indenture”), between the Company and the Trustee.  The
terms of this Debenture include those stated in the Indenture and in the First
Supplemental Indenture and those made part of the Indenture by reference to the
Trust Indenture Act of 1939, as in effect on the date of the
Indenture.  Reference is hereby made to the Indenture and all further
supplemental indentures thereto for a statement of the respective rights,
limitation of rights, duties and immunities thereunder of the Company, the
Trustee and the Holders and of the terms upon which the Debentures are, and are
to be, authenticated and delivered.




The Debentures are
a Series of Securities issued or to be issued by the Company under the
Indenture, and this Series is limited in aggregate principal amount to
$100,000,000.  The Indenture provides that the Securities of the
Company referred to therein (“Securities”), including the Debentures, may be
issued in one or more Series, which different Series may be issued in such
aggregate principal amounts and on such terms (including, but not limited to,
terms relating to interest rate or rates, provisions for determining such
interest rate or rates and adjustments thereto, maturity, redemption (optional
and mandatory), sinking fund, covenants and Events of Default) as may be
provided in or pursuant to the Authorizing Resolution and/or supplemental
indenture (if any) relating to the several Series.




This Debenture may
not be redeemed at the option of the Company prior to its Maturity
Date.




On June 15, 2007,
or if such date is not a Business Day, then the next succeeding Business Day
(the “Redemption Date”), each Holder of Debentures will have the right (the
“Redemption Right”) to require the Company to redeem all or any part (equal to
$1,000 in principal amount or an integral multiple thereof) of such Holder’s
Debentures for cash at a redemption price equal to 100% of the aggregate
principal amount thereof plus accrued and unpaid interest, if any, to the
Redemption Date.




On or prior to
March 30, 2007, the Company will mail a notice to each Holder staring that (a)
in order for a Holder to exercise the Redemption Right, the Holder must
surrender the Debentures in respect of which the Redemption Right is being
exercised, together with the form entitled “Option of Holder to Elect Redemption
on June 15, 2007” set forth below, duly completed, or transfer such Debentures
by book-entry, to the Trustee during the period from April 15, 2007 and prior to
5:00 p.m.  (New York City time) on May 15, 2007 (or if such date is
not a Business Day, then the next succeeding Business Day), (b) any
election on the part of a Holder to exercise the Redemption Right effected in
accordance with the foregoing shall be irrevocable on the part of the Holder and
may not be withdrawn, (c) Holders whose Debentures are being redeemed only
in part will be issued new Debentures equal in principal amount to the
unredeemed portion of the Debentures surrendered, which unredeemed portion must
be equal to $1,000 in principal amount or an integral multiple thereof, and
(d) unless the Company defaults in the payment of principal and accrued
interest on the Debentures to be redeemed on the Redemption Date, interest on
such Debentures will cease to accrue on the Redemption Date.  The
Company will comply with the requirements of Rule 14e-l under the Securities
Exchange Act of 1934, as amended, and any other securities laws and regulations
thereunder to the extent such laws and regulations are applicable to the
redemption of the Debentures pursuant to the Redemption Right.




If an Event of
Default shall occur and be continuing, the principal of all the Debentures may
be declared due and payable in the manner and with the effect provided in the
Indenture.




The Indenture
permits, with certain exceptions as therein provided, the amendment thereof and
the modification of the rights and obligations of the Company and the rights of
the Holders of the Debentures under the Indenture at any time by the Company and
the Trustee with the consent of the Holders of a majority in aggregate principal
amount of the Securities affected thereby, voting as a single class (which may
include the Debentures), at the time outstanding.  The Indenture also
contains provisions permitting the Holders of specified percentages in aggregate
principal amount of the Securities at the time outstanding to waive compliance
by the Company with certain provisions of the Indenture and certain past
defaults under the Indenture and their consequences.  Any such consent
or waiver by the Holder of this Debenture shall be conclusive and binding upon
such Holder and upon all future Holders of this Debenture and of any Debenture
issued upon the registration of transfer hereof or in exchange herefor or in
lieu hereof, whether or not notation of such consent or waiver is made upon this
Debenture.




The Indenture
provides that no Holder may pursue any remedy under the Indenture unless the
Trustee shall have failed to act after notice of an Event of Default and written
request by Holders of at least 25% in principal amount of the Securities of the
applicable Series and the offer to the Trustee of indemnity satisfactory to it;
however, such provision does not affect the right to sue for enforcement of any
overdue payment on any Debenture.




No reference herein
to the Indenture or the First Supplemental Indenture and no provision of this
Debenture or of the indenture or the First Supplemental Indenture shall alter or
impair the obligation of the Company, which is absolute and unconditional, to
pay the principal of and interest on this Debenture at the times, places and
rates, and in the coin or currency, herein prescribed.




As provided in the
Indenture and subject to certain limitations therein set forth, the transfer of
this Debenture is registrable in the Security Register upon surrender of this
Debenture for registration of transfer at the agency of the Company provided for
that purpose duly endorsed by, or accompanied by a written instrument of
transfer in substantially the form accompanying this Debenture duly executed by,
the Holder hereof or his attorney duly authorized in writing, and thereupon one
or more new Debentures, of authorized denominations and for the same aggregate
principal amount, will be issued to the designated transferee or
transferees.




The Debentures are
issuable only in registered form without coupons in denominations of $1,000 and
any integral multiple thereof.  As provided in the Indenture and
subject to certain limitations set forth therein and on the face of this
Debenture, the Debentures are exchangeable for a like aggregate principal amount
of Debentures of a different authorized denomination, as requested by the Holder
surrendering the same.




No service charge
shall be made for any such registration of transfer or exchange, but the Company
may require payment of a sum sufficient to cover any transfer tax or similar
governmental charge payable in connection therewith (other than any such
transfer tax or similar governmental charge payable upon exchanges pursuant to
Section 2.11, 3A.08 or 9.05 of the Indenture in which case such transfer taxes
or similar governmental charges shall be paid by the Company).




Prior to due
presentment of this Debenture for registration of transfer, the Company, the
Trustee and any agent of the Company or the Trustee may treat the person in
whose name this Debenture is registered as the owner hereof for all purposes,
whether or not this Debenture be overdue, and neither the Company, the Trustee
nor any such agent shall be affected by notice to the contrary.




All terms used in
this Debenture which are defined in the Indenture shall have the meanings
assigned to them in the Indenture.




Customary
abbreviations may be used in the name of a Debenture holder or any assignee,
such as: TEN COM (= tenants in common), TEN ENT(= tenants by the entireties), JT
TEN (= joint tenants with right of survivorship and not as tenants in common),
CUST (= Custodian) and U/G/M/A (= Uniform Gifts to Minors Act).




The Company will
furnish to any Holder of record of a Debenture, upon written request, without
charge, a copy of the Indenture and the First Supplemental
Indenture.  Requests may be made to: Colorado Interstate Gas Company,
c/o The Coastal Corporation, Coastal Tower, Nine Greenway Plaza, Houston, Texas
77046-0995, Attention: Corporate Secretary.







A-


ASSIGNMENT
FORM




If you the Holder
wants to assign this Debenture, fill in the form below and have your signature
guaranteed:




I or we assign and
transfer this Debenture to:







(Print
or type name, address and zip code and


social
security or tax ID number of assignees)




and irrevocably
appoint




agent to transfer
this Debenture on the books of the Company.  The agent may substitute
another to act for him.




Dated:                                                              Signed:








(Sign
exactly as name appears on the other side of this Debenture)








Signature
Guarantee:








NOTICE:                      Signature(s)
must be guaranteed by a member firm of the New York Stock Exchange or a
commercial bank or trust company.









A-


OPTION
OF HOLDER TO ELECT REDEMPTION ON JUNE 15, 2007




If you elect to
have this Debenture redeemed by the Company on June 15, 2007, check the
box:  ⁪




If you elect to
have only part of this Debenture redeemed by the Company on June 15, 2007, state
the amount (multiples of $1,000 only).




$





Dated:                                                              Signed:








(Sign
exactly as name appears on the other side of this Debenture)








Signature
Guarantee: