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                                                                    Exhibit 4.3

                             SUPPLEMENTAL INDENTURE


         SUPPLEMENTAL INDENTURE (this Supplemental Indenture) dated as of
February 25, 1998, between the undersigned Subsidiary Guarantors(individually,
the "Subsidiary Guarantor"), a subsidiary of Belco Oil & Gas Corp. (or its
successor), a company incorporated under the laws of the State of Nevada (the
"Company"), and The Bank of New York, a New York banking corporation, as
trustee under the indenture referred to below (the "Trustee").

                              W I T N E S S E T H

         WHEREAS, the Company has heretofore executed and delivered to the
Trustee an indenture (the "Indenture"), dated as of September 23, 1997,
providing for the issuance of an aggregate principal amount at maturity of
$150,000,000 of 8-7/8% Senior Subordinated Notes due 2007 (the "Notes");

         WHEREAS, Section 4.16 of the Indenture provides that, under certain
circumstances, the Company is required to cause the Subsidiary Guarantor to
execute and deliver to the Trustee a Subsidiary Guarantee on the terms and
conditions set forth herein; and

         WHEREAS, pursuant to Section 9.1 of the Indenture, the Company, the
Subsidiary Guarantor and the Trustee are authorized to execute and deliver this
Supplemental Indenture.

         NOW, THEREFORE, in consideration of the foregoing and for other good
and valuable consideration, the receipt of which is hereby acknowledged, the
Company, the Subsidiary Guarantor and the Trustee mutually covenant and agree
for the equal and ratable benefit of the holders of the Notes as follows:


         1.       CAPITALIZED TERMS.

                  Capitalized terms used herein without definition shall have
the meanings assigned to them in the Indenture.

         2.       INDENTURE PROVISION PURSUANT TO WHICH GUARANTEE IS
GIVEN.

                  This Supplemental Indenture is being executed and delivered
pursuant to Sections 4.16 and 9.1 of the Indenture.

         3.       AGREEMENTS TO GUARANTEE.

                  The Subsidiary Guarantor hereby agrees as follows:

                  (a) The Subsidiary Guarantor, jointly and severally with all
other Subsidiary Guarantors, if any, unconditionally guarantees to each Holder
of a Note authenticated and delivered by the Trustee and to the Trustee and its
successors and assigns, regardless of the validity and enforceability of the
Indenture, the Notes and the obligations of the Company under the Indenture and
the Notes, that:



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                           (i)  the principal of, premium, if any, and interest
on the Notes shall be promptly paid in full when due, whether at maturity, by
acceleration, redemption or otherwise, and interest on the overdue principal
of, premium, if any, and interest on the Notes, to the extent lawful, and all
other obligations of the Company to the Holders or the Trustee thereunder shall
be promptly paid in full, all in accordance with the terms thereof; and

                           (ii) in case of any extension of time for payment or
renewal of any Notes or any of such other obligations, that the same shall be
promptly paid in full when due in accordance with the terms of the extension or
renewal, whether at maturity, by acceleration or otherwise.

         Notwithstanding the foregoing, in the event that this Subsidiary
Guarantee would constitute or result in a violation of any applicable
fraudulent conveyance or similar law of any relevant jurisdiction, the
liability of the Subsidiary Guarantor under this Supplemental Indenture and its
Subsidiary Guarantee shall be limited to such amount as will not, after giving
effect thereto, and to all other liabilities of the Subsidiary Guarantor,
result in such amount constituting a fraudulent transfer or conveyance.

         4.       SUBORDINATION.

                  The Subsidiary Guarantor agrees, and each Holder by accepting
a Note agrees, that (a) the obligations of the Subsidiary Guarantor under this
Subsidiary Guarantee are subordinated in right of payment to the prior payment
in full (when due) of all existing and future Guarantor Senior Indebtedness of
the Subsidiary Guarantor, including without limitation any guarantee by the
Subsidiary Guarantor of the Indebtedness under the New Credit Facility or of
any Senior Debt of the Company or of any Guarantor Senior Indebtedness of any
other Subsidiary Guarantor, to the extent and in the matter provided in Article
10 (including, without limitation, Section 10.12) of the Indenture (as if the
Subsidiary Guarantor were the Company for purposes of such Article 10 and all
defined terms used therein, and the Guarantor Senior Indebtedness of the
Guarantor were Senior Debt), and this Subsidiary Guarantee is made subject to
such provisions (which are hereby incorporated herein by reference), and (b)
such subordination is for the benefit of and enforceable by the holders of
Guarantor Senior Indebtedness of the Subsidiary Guarantor.

         5.       EXECUTION AND DELIVERY OF SUBSIDIARY GUARANTEE.


                  (a) To evidence its Subsidiary Guarantee set forth in this
Supplemental Indenture, the Subsidiary Guarantor hereby agrees that a notation
of such Subsidiary Guarantee substantially in the form of Annex A hereto shall
be endorsed by an officer of such Subsidiary Guarantor on each Note
authenticated and delivered by the Trustee after the date hereof.

                  (b) Notwithstanding the foregoing, the Subsidiary Guarantor
hereby agrees that its Subsidiary Guarantee set forth herein shall remain in
full force and effect notwithstanding any failure to endorse on each Note a
notation of such Subsidiary Guarantee.

                  (c) If an officer whose signature is on this Supplemental
Indenture or on the Subsidiary Guarantee no longer holds that office at the
time the Trustee authenticates the Note on which a Subsidiary Guarantee is
endorsed, the Subsidiary Guarantee shall be valid nevertheless.



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                  (d) The delivery of a Note by the Trustee, after the
authentication thereof under the Indenture, shall constitute due delivery of
the Subsidiary Guarantee set forth in this Supplemental Indenture on behalf of
the Subsidiary Guarantor.

                  (e) The Subsidiary Guarantor hereby agrees that its
obligations hereunder shall be unconditional, regardless of the validity,
regularity or enforceability of the Notes or the Indenture, the absence of any
action to enforce the same, any waiver or consent by any Holder of the Notes
with respect to any provisions hereof or thereof, the recovery of any judgment
against the Company, any action to enforce the same or any other circumstance
which might otherwise constitute a legal or equitable discharge or defense of a
guarantor.

                  (f) The Subsidiary Guarantor hereby waives diligence,
presentment, demand of payment, filing of claims with a court in the event of
insolvency or bankruptcy of the Company, any right to require a proceeding
first against the Company, protest, notice and all demands whatsoever and
covenants that its Subsidiary Guarantee made pursuant to this Supplemental
Indenture will not be discharged except by complete performance of the
obligations contained in the Notes and the Indenture or pursuant to Section
6(b) of this Supplemental Indenture.

                  (g) If the Trustee or any Holder has instituted any
proceeding to enforce any right or remedy under this Supplemental Indenture or
under the Subsidiary Guarantee made pursuant to this Supplemental Indenture and
such proceeding has been discontinued or abandoned for any reason, or has been
determined adversely to the Trustee or to such Holder, then, and in every such
case, subject to any determination in such proceeding, the Subsidiary
Guarantor, the Trustee and the Holders shall be restored severally and
respectively to their former positions hereunder and thereafter all rights and
remedies of the Subsidiary Guarantor, the Trustee and the Holders shall
continue as though no such proceeding had been instituted.

                  (h) The Subsidiary Guarantor hereby waives and will not in
any manner whatsoever claim or take the benefit or advantage of, any rights of
reimbursement, indemnity or subrogation or any other rights against the Company
or any other Subsidiary Guarantor as a result of any payment by such Subsidiary
Guarantor under its Subsidiary Guarantee. The Subsidiary Guarantor further
agrees that, as between the Subsidiary Guarantors, on the one hand, and the
Holders and the Trustee, on the other hand:

                           (i)  the maturity of the obligations guaranteed 
hereby may be accelerated as provided in Article 6 of the Indenture for the
purposes of the Subsidiary Guarantee made pursuant to this Supplemental
Indenture, notwithstanding any stay, injunction or other prohibition preventing
such acceleration in respect of the obligations guaranteed hereby; and

                           (ii)  in the event of any declaration of acceleration
of such obligations as provided in such Article 6, such obligations (whether or
not due and payable) shall forthwith become due and payable by the Subsidiary
Guarantor for the purpose of the Subsidiary Guarantee made pursuant to this
Supplemental Indenture.

                  (i) The Subsidiary Guarantor shall have the right to seek
contribution from any other non-paying Subsidiary Guarantor, if any, so long as
the exercise of such right does not impair the rights of the Holders under the
Subsidiary Guarantee made pursuant to this Supplemental Indenture.



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                  (j) The Subsidiary Guarantor covenants (to the extent that it
may lawfully do so) that it will not at any time insist upon, or plead, or in
any manner whatsoever claim or take the benefit or advantage of, any stay,
extension or usury law wherever enacted, now or at any time hereafter in force,
which may affect the covenants or the performance of the Indenture or this
Subsidiary Guarantee; and the Subsidiary Guarantor (to the extent that it may
lawfully do so) hereby expressly waives all benefit or advantage of any such
law, and covenants that it will not hinder, delay or impede the execution of
any power herein granted to the Trustee, but will suffer and permit the
execution of every such power as though no such law had been enacted.

         6.       SUBSIDIARY GUARANTOR MAY CONSOLIDATE, ETC. ON CERTAIN
TERMS

                  (a) Except as set forth in Articles 4 and 5 of the Indenture,
nothing contained in the Indenture, this Supplemental Indenture or in the Notes
shall prevent any consolidation or merger of the Subsidiary Guarantor with or
into the Company or any other Subsidiary Guarantor or shall prevent any
transfer, sale or conveyance of the property of the Subsidiary Guarantor as an
entirety or substantially as an entirety, to the Company or any other
Subsidiary Guarantor.

                   (b) Except as set forth in Article 5 of the Indenture, upon
the sale or disposition of all of the Capital Stock of the Subsidiary Guarantor
by the Company or the Subsidiary of the Company, or upon the consolidation or
merger of the Subsidiary Guarantor with or into any Person, or the sale of all
or substantially all of the assets of the Subsidiary Guarantor (in each case,
other than to an Affiliate of the Company), such Subsidiary Guarantor shall be
deemed automatically and unconditionally released and discharged from all
obligations under this Subsidiary Guarantee without any further action required
on the part of the Trustee or any Holder if no Default shall have occurred and
be continuing; provided, that in the event of an Asset Sale, the Net Proceeds
therefrom are treated in accordance with Section 4.10 of the Indenture. Except
with respect to transactions set forth in the preceding sentence, the Company
and the Subsidiary Guarantor covenant and agree that upon any such
consolidation, merger or transfer of assets, the performance of all covenants
and conditions of this Supplemental Indenture to be performed by such
Subsidiary Guarantor shall be expressly assumed by supplemental indenture
satisfactory in form to the Trustee, by the corporation formed by such
consolidation, or into which the Subsidiary Guarantor shall have merged, or by
the corporation which shall have acquired such property. Upon receipt of an
Officer's Certificate of the Company or the Subsidiary Guarantor, as the case
may be, to the effect that the Company or such Subsidiary Guarantor has
complied with the first sentence of this Section 6(b), the Trustee shall
execute any documents reasonably requested by the Company or the Subsidiary
Guarantor, at the cost of the Company or such Subsidiary Guarantor, as the case
may be, in order to evidence the release of such Subsidiary Guarantor from its
obligations under its Guarantee endorsed on the Notes and under the Indenture
and this Supplemental Indenture.

         7.       RELEASES UPON RELEASE OF GUARANTEE OF GUARANTEED
INDEBTEDNESS.

                  Concurrently with the release or discharge of the Subsidiary
Guarantor's guarantee of the payment of (i) notes and other obligations under
that certain Indenture dated as of March 18, 1996, among Belco Oil & Gas Corp.
as successor by merger to Coda Energy, Inc. (as issuer) and Diamond Energy
Operating Company, Taurus Energy Corp. and Electra Resources, Inc. (as
guarantors) and Chase Bank of Texas, N.A. (formerly known as Texas Commerce
Bank, N.A.) (as trustee) and (ii) a Credit Agreement among Belco Oil & Gas
Corp. (as borrower), The Chase 


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Manhattan Bank (as administrative agent), and the lender party thereto
("Guaranteed Debt") (other than a release or discharge by or as a result of
payment under such guarantee of Guaranteed Indebtedness), the Subsidiary
Guarantor shall be automatically and unconditionally released and relieved of
its obligations under this Supplemental Indenture and its Subsidiary Guarantee
made pursuant to Section 5 of this Supplemental Indenture. Upon delivery by the
Company to the Trustee of an Officer's Certificate to the effect that such
release or discharge has occurred, the Trustee shall execute any documents
reasonably required in order to evidence the release of the Subsidiary
Guarantor from its obligations under this Supplemental Indenture and its
Subsidiary Guarantee made pursuant hereto; provided such documents shall not
affect or impair the rights of the Trustee and Paying Agent under Section 7.7
of the Indenture.

         8.       NEW YORK LAW TO GOVERN.

                  The substantive law of the State of New York shall govern and
be used to construe this Supplemental Indenture.

         9.       COUNTERPARTS.

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

         10.      RECITALS, ETC.

                  The recitals herein contained are made by the Company and not
by the Trustee, and the Trustee assumes no responsibility for the correctness
thereof. The Trustee makes no representations as to the validity or sufficiency
of this Supplemental Indenture.

         11.      EFFECT OF HEADINGS.

                  The Section headings herein are for convenience only and
shall not effect the construction hereof.



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         IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture to be duly executed and attested, all as of the date first above
written.


                                      BELCO OIL & GAS CORP.


                                      By:   /s/         Robert A. Belfer
                                         ---------------------------------------
                                         Name:          Robert A. Belfer
                                         Title:         Chairman and CEO


                                      CODA ENERGY, INC., a Nevada corporation


                                      By:     /s/       Grant W. Henderson
                                         ---------------------------------------
                                         Name:          Grant W. Henderson
                                         Title:         President and CFO


                                      DIAMOND ENERGY OPERATING COMPANY, an
                                        Oklahoma corporation


                                      By:    /s/        Grant W. Henderson
                                         ---------------------------------------
                                         Name:          Grant W. Henderson
                                         Title:         Vice President


                                      ELECTRA RESOURCES, INC., a Texas
                                       corporation


                                      By:     /s/       Grant W. Henderson
                                         ---------------------------------------
                                         Name:          Grant W. Henderson
                                         Title:         President


                                      BELCO OPERATING CORP., a Delaware
                                       corporation


                                      By:   /s/         Laurence D. Belfer
                                         ---------------------------------------
                                         Name:          Laurence D. Belfer
                                         Title:         President & COO





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                                      BELCO ENERGY L.P., a Delaware limited
                                       partnership

                                      By:     BELCO OPERATING CORP., its general
                                               partner


                                      By:    /s/        Laurence D. Belfer
                                         ---------------------------------------
                                         Name:          Laurence D. Belfer
                                         Title:         President & COO


                                      GIN LANE COMPANY, a Delaware
                                       corporation


                                      By:   /s/         Robert A. Belfer
                                         ---------------------------------------
                                         Name:          Robert A. Belfer
                                         Title:         President


                                      FORTUNE CORP., a Texas corporation


                                      By: /s/           Robert A. Belfer
                                         ---------------------------------------
                                         Name:          Robert A. Belfer
                                         Title:         President


                                      BOG WYOMING LLC, a Wyoming limited
                                       liability corporation

                                      By:      BELCO OPERATING CORP., managing
                                                member


                                      By:   /s/         Laurence D. Belfer
                                         ---------------------------------------
                                         Name:          Laurence D. Belfer
                                         Title:         President & COO




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                                      BELCO FINANCE CO., a Wyoming corporation


                                      By:   /s/         Robert A. Belfer
                                         ---------------------------------------
                                         Name:          Robert A. Belfer
                                         Title:         President


                                      THE BANK OF NEW YORK,
                                        as Trustee


                                      By:    /s/     Lucille Firrincieli
                                         ---------------------------------------
                                         Name:          Lucille Firrincieli
                                         Title:


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                       ANNEX A TO SUPPLEMENTAL INDENTURE

                FORM OF NOTATION OF SUBSIDIARY GUARANTEE ON NOTE


         Each Subsidiary Guarantor (as defined in the Indenture) has jointly
and severally unconditionally guaranteed (a) the due and punctual payment of
the principal of, premium, if any, and interest on the Notes, whether at Stated
Maturity or an Interest Payment Date, by acceleration, call for redemption or
otherwise, (b) the due and punctual payment of interest on the overdue
principal and premium of, and interest, to the extent lawful, on the Notes and
(c) that in case of any extension of time of payment or renewal of any Notes or
any of such other obligations, the same will be promptly paid in full when due
in accordance with the terms of the extension of renewal, whether at stated
maturity, by acceleration or otherwise.

         Notwithstanding the foregoing, in the event that the Subsidiary
Guarantee would constitute or result in a violation of any applicable
fraudulent conveyance or similar law of any relevant jurisdiction, the
liability of the Subsidiary Guarantor under its Subsidiary Guarantee shall be
limited to such amount as will not, after giving effect thereto, and to all
other liabilities of the Subsidiary Guarantor, result in such amount
constituting a fraudulent transfer or conveyance.

         This Subsidiary Guarantee shall not be valid or obligatory for any
purpose until the certificate of authentication on the Note upon which the
Subsidiary Guarantee is noted shall have been executed by the Trustee under the
Indenture by the manual signature of one of its authorized signatories.

Dated: ____________________, _______



                                           [SUBSIDIARY GUARANTOR]


                                           By:_________________________________
                                              Name:
                                              Title: