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                                                                    EXHIBIT 4.15


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                     AMERICAN COMMUNICATIONS SERVICES, INC.

                                  $220,000,000

                          13 3/4% SENIOR NOTES DUE 2007

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                             SUPPLEMENTAL INDENTURE

                          Dated as of February 27, 1998

                                       to

                                    INDENTURE

                            Dated as of July 23, 1997

                        --------------------------------

                            THE CHASE MANHATTAN BANK,

                                     Trustee

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                  SUPPLEMENTAL INDENTURE (this "Supplemental Indenture"), dated
as of February 27, 1998, by and between AMERICAN COMMUNICATIONS SERVICES, INC.,
a Delaware corporation (the "Company"), having its principal office at 131
National Business Parkway, Suite 100, Annapolis Junction, Maryland 20701, and
The Chase Manhattan Bank (formerly known as Chemical Bank), a New York banking
corporation, as trustee (the "Trustee") under the Indenture (as defined below),
having its Corporate Trust Office at 450 West 33rd Street, New York, New York
10001-2697. Capitalized terms used but not defined herein shall have the
meanings assigned to such terms in the Indenture.

                  WHEREAS, the Company and the Trustee previously duly executed,
and the Company duly delivered to the Trustee, an Indenture dated as of July 23,
1997 (the "Indenture"), relating to $220,000,000 aggregate principal amount of
the Company's 13 3/4% Senior Notes due 2007 (the "Notes");

                  WHEREAS, pursuant to Section 9.02 of the Indenture, the
Company and the Trustee have obtained the consent of the Holders of not less
than a majority in principal amount of the outstanding Notes to the amendments
made hereby;

                  WHEREAS, the Board of Directors of the Company has authorized
the execution of this Supplemental Indenture and its delivery to the Trustee;

                  WHEREAS, the Company has delivered an Officers' Certificate
and Opinion of Counsel to the Trustee pursuant to Section 9.07 of the Indenture;
and

                  WHEREAS, all other actions necessary to make this Supplemental
Indenture a legal, valid and binding obligation of the parties hereto in
accordance with its terms and the terms of the Indenture have been performed;

                  NOW, THEREFORE, in consideration of the promises contained
herein and for other good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, the Company and the Trustee hereby
mutually covenant and agree for the equal and proportionate benefit of all
Holders of the Notes as follows:

                                        I
                                   AMENDMENTS

                  Upon execution of this Supplemental Indenture, the terms of
the Notes and the Indenture shall be amended as follows:

                  I.1. Section 1.01 of the Indenture shall be amended as
follows:

                  (a) by deleting clause (x) of the definition of "Permitted
Investments" and substituting in lieu thereof the following:

                  "(x)     Investments funded or financed with Purchase Money
         Debt; and


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                     (xi)          Investments in existence at the Issue Date.";

                  (b) by deleting clause (v) of the definition of "Permitted
Liens" and substituting in lieu thereof the following:

                  "(v)  Liens to secure Indebtedness permitted to be incurred
         under Section 4.09(b)(i) or Section 4.09(b)(ix) hereof;" and

                  (c) by adding the following definitions in appropriate
alphabetical order:

                  "`Debt to Capital Ratio' means, as at any date of
         determination, the ratio of (i) the amount of Indebtedness of the
         Company and its Restricted Subsidiaries then outstanding on a
         consolidated basis as at the date of determination to (ii) the capital
         of the Company and its Restricted Subsidiaries on a consolidated basis
         as at such date. For purposes of this calculation, "capital" shall mean
         stockholders' equity (deficit), except that all Preferred Stock shall
         be included and retained earnings (deficit) shall be excluded, each as
         determined in accordance with GAAP."

                  "`Purchase Money Debt' means Indebtedness of the Company
         (including Acquired Indebtedness and Indebtedness represented by
         Capital Lease Obligations, mortgage financings and purchase money
         obligations) incurred for the purpose of financing all or any part of
         the cost of construction, acquisition or improvement by the Company or
         any Subsidiary of the Company or any joint venture of any
         Telecommunications Assets of the Company, any Subsidiary of the Company
         or any joint venture, and including any related notes, Guarantees,
         collateral documents, instruments and agreements executed in connection
         therewith, as the same may be amended, supplemented, modified or
         restated from time to time."


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                  I.2. Section 4.09(a) of the Indenture shall be amended by
deleting such paragraph in its entirety and substituting in lieu thereof the
following:

                  "(a) The Company shall not, and shall not permit its
         Restricted Subsidiaries to, directly or indirectly, incur any
         Indebtedness (including Acquired Indebtedness), and the Company shall
         not issue any Disqualified Stock or permit any of its Restricted
         Subsidiaries to issue any Disqualified Stock or Preferred Stock;
         provided that the Company may incur Indebtedness or issue Disqualified
         Stock if, after giving effect to such issuance or incurrence on pro
         forma basis, (i) the Debt to EBITDA Ratio of the Company does not
         exceed 5.5x in the case of any issuance or incurrence on or before
         November 1, 1998, or 5.0x in the case of any issuance or incurrence
         thereafter or (ii) the Debt to Capital Ratio as of the most recent
         available quarterly or annual balance sheet, after giving pro forma
         effect to the incurrence of such Indebtedness and any other
         Indebtedness incurred since such balance sheet date and the receipt and
         application of the proceeds thereof, does not exceed 2.0x; provided
         that in determining the amount of Indebtedness the Company may incur in
         reliance upon the alternative set forth in clause (ii), $100,000,000
         shall be subtracted from such amount."

                  I.3. Section 4.09(b) of the Indenture shall be amended as
follows:

                  (a) by deleting in its entirety clause (ix) thereof and
substituting in lieu thereof the following:

                  "(ix)    Purchase Money Debt, provided that the amount of such
         Purchase Money Debt does not exceed the cost of the construction,
         acquisition or improvement of the applicable Telecommunications Assets;
         provided, however that the Company may not incur, as of any date of
         determination, Purchase Money Debt in excess of 50% of the amount of
         accreted unsecured Indebtedness of the Company and its Restricted
         Subsidiaries on a consolidated basis as at such date (the "Purchase
         Money Debt Allowance"); provided further, however that in calculating
         such Purchase Money Debt Allowance, incurrences of Purchase Money Debt
         consisting of (A) Capital Lease Obligations resulting from the
         conversion of operating leases in an amount not to exceed $36,000,000
         and (B) Indebtedness incurred pursuant to Section 4.09(b)(i) shall not
         be included;"; and

                  (b) by deleting "$428,634" at the end of clause (xiv) thereof
and substituting in lieu thereof, "$10,000,000".


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                                       II
                                 MISCELLANEOUS

                  II.1. For all purposes of this Supplemental Indenture, except
as otherwise herein expressly provided or unless the context otherwise requires:
(A) the terms and expressions used herein shall have the same meanings as
corresponding terms and expressions used in the Indenture and (B) the words
"herein," "hereof" and "hereby" and other words of similar import used in this
Supplemental Indenture refer to this Supplemental Indenture as a whole and not
any particular Article, Section or other subdivision.

                  II.2. Upon execution of this Supplement Indenture, the
Indenture shall be modified in accordance herewith, but except as expressly
amended hereby, the Indenture is in all respects ratified and confirmed and all
the terms, conditions and provisions thereof shall remain in full force and
effect.

                  II.3. Upon execution, this Supplemental Indenture shall form a
part of the Indenture and the Supplemental Indenture and the Indenture shall be
read, taken and construed as one and the same instrument for all purposes, and
every holder of Notes heretofore or hereafter authenticated and delivered under
the Indenture shall be bound hereby.

                  II.4. This Supplemental Indenture shall become effective as of
the date first above written.

                  II.5. The Trustee accepts the amendment to the Indenture
effected by this Supplemental Indenture and agrees to execute the trust created
by the Indenture, as hereby amended, but only upon the terms and conditions set
forth in the Indenture, as hereby amended, including the terms and provisions
defining and limiting the liabilities and responsibilities of the Trustee, which
terms and provisions shall in like manner define and limit the Trustee's
liabilities in the performance of the trust created by the Indenture, as hereby
amended. Without limiting the generality of the foregoing, the Trustee has no
responsibility for the correctness of the recitals of fact herein contained
which shall be taken as the statements of the Company and makes no
representations as to the validity or sufficiency of this Supplemental
Indenture, except as to the due and valid execution hereof by the Trustee, and
shall incur no liability or responsibility in respect of the validity thereof.
The Trustee's execution of this Supplemental Indenture should not be construed
to be an approval or disapproval of the advisability of the amendments to the
Indenture provided herein.

                  II.6. THIS SUPPLEMENTAL INDENTURE SHALL BE GOVERNED BY, AND
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK APPLICABLE TO
AGREEMENTS MADE AND TO BE PERFORMED ENTIRELY IN SAID STATE.


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                  II.7. This Supplemental Indenture may be executed in any
number of counterparts, each of which when so executed shall be deemed to be an
original, and all of such counterparts shall together constitute one and the
same instrument.


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

                                          AMERICAN COMMUNICATIONS SERVICES, INC.

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

                                          THE CHASE MANHATTAN BANK, as Trustee

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