BLACK HILLS CORP
S-3, 1994-06-28
ELECTRIC SERVICES
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<PAGE>
 
As Filed with the Securities and Exchange Commission on         Registration No.
 
================================================================================
 
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                                 ------------
 
                                    FORM S-3
 
                            REGISTRATION STATEMENT
                                     Under
                          THE SECURITIES ACT OF 1933
 
                                 ------------
 
                            BLACK HILLS CORPORATION
             (Exact name of Registrant as specified in its charter)
 
           South Dakota                                       46-0111677
   (State or other jurisdiction of                          (I.R.S. Employer
   incorporation or organization)                          Identification No.)
 
                                ------------
 
       625 Ninth Street, P. O. Box 1400, Rapid City, South Dakota 57709
                                (605) 348-1700
  (Address, including zip code, and telephone number, including area code, of
                   registrant's principal executive offices)
 
                                ------------
 
                                Dale E. Clement
                        Senior Vice President--Finance
                            Black Hills Corporation
                               625 Ninth Street
                        Rapid City, South Dakota 57701
                                (605) 348-1700
 
 (Name, address, including zip code, and telephone number, including area code,
                             of agent for service)
 
                                 ------------
 
                                   Copies to
 
           David E. Morrill                         Howard G. Godwin, Jr.  
         Morrill Brown & Thomas                          Brown & Wood
    625 Ninth Street, P. O. Box 8108                One World Trade Center
   Rapid City, South Dakota 57709-8108             New York, N.Y. 10048-0557
 
                                 ------------
 
         Approximate date of commencement of proposed sale to public:
  From time to time after the effective date of the Registration Statement as
                       determined by market conditions.
 
  If the only securities being registered on this Form are being offered
pursuant to dividend or interest reinvestment plans, please check the following
box. [_]
 
  If any of the securities being registered on this Form are to be offered on a
delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933, other than securities offered only in connection with dividend or
interest reinvestment plans, check the following box. [X]
 
                        CALCULATION OF REGISTRATION FEE
 
================================================================================
<TABLE>
<CAPTION>
                                         Proposed         Proposed
Title of Each Class                      Maximum          Maximum        Amount of
   of Securities         Amount To Be Offering Price     Aggregate      Registration
 To Be Registered         Registered   Per Unit (1)  Offering Price (1)     Fee
- ------------------------------------------------------------------------------------
<S>                      <C>          <C>            <C>                <C>
First Mortgage Bonds.... $100,000,000      100%         $100,000,000      $34,483
</TABLE>
================================================================================
 
  (1) Estimated in accordance with Rule 457 solely for the purpose of
      calculating the registration fee.
 
                                 ------------
 
  The registrant hereby amends this Registration Statement on such date or
dates as may be necessary to delay its effective date until the registrant
shall file a further amendment which specifically states that this Registration
Statement shall thereafter become effective in accordance with Section 8(a) of
the Securities Act of 1933 or until the Registration Statement shall become
effective on such date as the Commission, acting pursuant to said Section 8(a),
may determine.
 
================================================================================
<PAGE>
 
                   SUBJECT TO COMPLETION, DATED JUNE 28, 1994
 
PROSPECTUS
- ----------
  
                                  $100,000,000
                            Black Hills Corporation
 
                              First Mortgage Bonds
 
                                  -----------
 
  Black Hills Corporation (the "Company") intends from time to time or at one
time, to issue up to $100,000,000 aggregate principal amount of its First
Mortgage Bonds in one or more series, on terms for each series to be determined
when the agreement to sell is made or at the time of sale, as the case may be.
For each issue of First Mortgage Bonds for which this Prospectus is being
delivered (the "Offered Bonds"), there will be an accompanying Prospectus
Supplement ("Prospectus Supplement") that will set forth the series
designation, aggregate principal amount of the issue, maturity, rates and times
of payment of interest, and redemption terms, if any, credit enhancement terms,
if any, and other special terms of each series of Offered Bonds offered
thereby, as well as any plans for application to list the Offered Bonds on a
stock exchange.
 
                                  -----------
 
 THESE SECURITIES HAVE NOT BEEN APPROVED  OR DISAPPROVED BY THE SECURITIES AND
  EXCHANGE  COMMISSION  OR  ANY  STATE  SECURITIES  COMMISSION  NOR  HAS  THE
   SECURITIES  AND EXCHANGE  COMMISSION OR  ANY STATE SECURITIES  COMMISSION
    PASSED   UPON  THE  ACCURACY  OR  ADEQUACY  OF  THIS   PROSPECTUS.  ANY
      REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.
 
                                  -----------
 
  The Company may sell the Offered Bonds through underwriters, dealers or
agents, or directly to one or more purchasers. The applicable Prospectus
Supplement will set forth the names of underwriters, dealers or agents, if any,
any applicable commissions or discounts and the net proceeds to the Company
from any such sale. See "Plan of Distribution" herein.
 
                                  -----------
 
                 The date of this Prospectus is June 28, 1994.
 
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+Information contained herein is subject to completion or amendment. A         +
+registration statement relating to these securities has been filed with the   +
+Securities and Exchange Commission. These securities may not be sold nor may  +
+offers to buy be accepted prior to the time the registration statement        +
+becomes effective. This prospectus shall not constitute an offer to sell or   +
+the solicitation of an offer to buy nor shall there be any sale of these      +
+securities in any State in which such offer, solicitation or sale would be    +
+unlawful prior to registration or qualification under the securities laws of  +
+any such State.                                                               +
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
<PAGE>
 
                             AVAILABLE INFORMATION
 
  The Company is subject to the informational requirements of the Securities
Exchange Act of 1934, as amended ("Exchange Act"), and in accordance therewith
files reports and other information with the Securities and Exchange Commission
("Commission"). Such reports and other information may be inspected and copied
at the offices of the Commission at 450 Fifth Street, N.W., Washington, DC
20549, Seven World Trade Center, New York, New York 10048 and 500 West Madison
Street, Chicago, Illinois 60661. Copies of this material can also be obtained
at prescribed rates from the Public Reference Section of the Commission at its
principal office at 450 Fifth Street, N.W., Washington, DC 20549. In addition,
the Common Stock of the Company is listed on The New York Stock Exchange, 20
Broad Street, New York, NY 10005, where reports and other information
concerning the Company may be inspected.
 
                INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE
 
  The following documents filed by the Company with the Commission are
incorporated by reference into this Prospectus.
 
    1. The Company's Annual Report on Form 10-K for the year ended December
       31, 1993.
 
    2. The Company's Quarterly Report on Form 10-Q for the quarter ended
       March 31, 1994.
 
    3. The Company's current report on Form 8-K dated June 7, 1994.
 
  All documents filed by the Company pursuant to Sections 13(a), 13(c), 14 or
15(d) of the Exchange Act after the date of this Prospectus and prior to the
termination of the offering made by this Prospectus shall be deemed to be
incorporated by reference in this Prospectus and to be made a part hereof from
the date of filing of such documents. Any statement contained in a document
incorporated by reference herein shall be deemed to be modified or superseded
for purposes of this Prospectus to the extent that a statement herein or in any
other subsequently filed document which also is or is deemed to be incorporated
by reference herein modifies or supersedes such statement. Any such statement
so modified or superseded shall not be deemed, except as so modified or
superseded, to constitute a part of this Prospectus.
 
  The Company hereby undertakes to provide without charge to each person,
including any beneficial owner, to whom a copy of this Prospectus has been
delivered, on the written or oral request of any such person, a copy of any or
all of the documents referred to above which have been or may be incorporated
in this Prospectus by reference, other than exhibits to such documents, unless
such exhibits are specifically incorporated by reference. Requests for such
copies should be directed to: Treasurer's Office, Black Hills Corporation, P.
O. Box 1400, Rapid City, SD 57709, or telephone (605) 348-1700.
 
                                  THE COMPANY
 
  The Company was incorporated under the laws of South Dakota in 1941 under the
name Black Hills Power and Light Company. In 1986 the Company changed its name
to Black Hills Corporation; however, the Company continues to operate its
investor-owned utility operations under the name of Black Hills Power and Light
Company. The Company's wholly-owned subsidiary, Wyodak Resources Development
Corp. ("Wyodak Resources"), a Delaware corporation, and Wyodak Resources'
wholly-owned subsidiary, Western Production Company, a Wyoming corporation, are
engaged in the business of coal mining and oil and gas production,
respectively.
 
                                       2
<PAGE>
 
                                USE OF PROCEEDS
 
  The proceeds from the sale of the Offered Bonds will be used to finance
capital expenditures, including the construction of Neil Simpson Unit #2, an 80
MW coal-fired electric generating plant now under construction adjacent to
Wyodak Resources' coal mine near Gillette, Wyoming, and to repay outstanding
short-term borrowings for such purposes and for general corporate purposes,
including the possible redemption, in whole or in part of the Company's
existing Bonds or other debt securities. The average interest rate of the
short-term indebtedness expected to be discharged with the proceeds is 4.8
percent as of May 31, 1994, and the maturity of such indebtedness is less than
12 months.
 
                       RATIO OF EARNINGS TO FIXED CHARGES
 
  The Company's ratio of earnings to fixed charges for each of the periods
indicated is as follows:
 
<TABLE>
<CAPTION>
           Year Ended December 31,           12 Months Ended   Quarter Ended
         ----------------------------------  March 31, 1994    March 31, 1994
         1989    1990   1991   1992   1993   ---------------   --------------
         ----    ----   ----   ----   ----
         <S>     <C>    <C>    <C>    <C>    <C>               <C>
         5.1     5.0    4.6    4.6    4.6          4.6              4.8
</TABLE>
 
  The ratio of earnings to fixed charges represents, on a pretax basis, the
number of times earnings cover fixed charges. Earnings consist of net income to
which has been added fixed charges and taxes based on income of the Company and
its subsidiaries. Fixed charges consist of interest charges and an interest
factor in rentals.
 
                        DESCRIPTION OF THE OFFERED BONDS
 
General
 
  The Offered Bonds will be issued in one or more series as fully registered
bonds, without coupons, under an Indenture of Mortgage and Deed of Trust, dated
as of September 1, 1941, between the Company and Chemical Bank (the "Trustee,"
successor by merger to the original and succeeding Trustees). The Indenture of
Mortgage and Deed of Trust, as supplemented by twenty-seven Supplemental
Indentures, is herein referred to as the "Mortgage." The Mortgage and the
twenty-seven Supplemental Indentures are filed with the Securities and Exchange
Commission with either the Registration Statement of which this Prospectus is a
part or with other previously filed registration statements as referenced.
 
  The summaries of the Mortgage herein contained do not purport to be complete
and are subject to the detailed provisions of the Mortgage. Capitalized terms
used herein which are not otherwise defined in this Prospectus shall have the
meanings ascribed thereto in the Mortgage. Whenever particular provisions of
the Mortgage or terms defined therein are referred to, such provisions or
definitions are incorporated by reference as a part of the statements made
herein, and such statements are qualified in their entirety by such reference.
References to article and section numbers in this description of the Offered
Bonds, unless otherwise indicated, are references to articles and section
numbers of the Mortgage.
 
  Reference is made to the Prospectus Supplement relating to any particular
series of Offered Bonds and any supplement thereto for the following terms,
which will be determined at the time or times of sale: (i) the designation of
such series; (ii) the aggregate principal amount of the Offered Bonds of such
series; (iii) the price (expressed as a percentage of the principal amount) at
which such Offered Bonds will be issued; (iv) the date(s) on which such Offered
Bonds mature; (v) the rate(s) per annum at which such Offered Bonds will bear
interest; (vi) the date(s) from which such interest will accrue, the dates on
which such interest will be payable, the date(s) on which such payments will
commence, and the record dates for such payments; (vii) the terms, if any, upon
which such Offered Bonds will be redeemable, whether at the option of the
Company or pursuant to any mandatory redemption provisions, including without
limitation redemption prices and any provisions for call protection; and (viii)
any other special terms.
 
                                       3
<PAGE>
 
Consent Bonds--Proposed Supplement
 
  The Company has prepared and filed as an exhibit to the Registration
Statement of which this Prospectus is a part the Proposed Twenty Eighth
Supplemental Indenture ("Proposed Supplement"), undated and unsigned. If
adopted, the Proposed Supplement would amend the Mortgage in various respects
as summarized in this Prospectus. The summaries of the Proposed Supplement
herein do not purport to be complete and are subject to the detailed provisions
of the Proposed Supplement. Whenever particular provisions of the Proposed
Supplement or terms defined therein are referred to, such provisions or
definitions are incorporated by reference as a part of the statements made
herein, and such statements are qualified in their entirety by such reference.
 
  The Proposed Supplement will not go into effect until the Company and the
Trustee execute the same. Under Article Eighteen of the Mortgage, the Trustee
does not have the authority to execute the Proposed Supplement without the
affirmative consent of the Holders of not less than 66 2/3 percent in principal
amount of the Bonds outstanding. The consent of the Bondholders is manifested
either at a meeting of Bondholders or, as provided by Section 18.11 of the
Mortgage, by written consent without the necessity of holding a meeting of the
Bondholders.
 
  The Company has determined that the Supplemental Indenture for each series of
the Offered Bonds and all additional Bonds hereafter issued under the Mortgage
until such time that the Proposed Supplement has been adopted will include a
consent provision to be substantially worded as follows:
 
   The Holders, including any successor Holders, of the Series      Bonds to
   be issued under the terms of this Supplemental Indenture, by becoming
   such Holders shall be deemed to have consented to the Twenty Eighth
   Supplemental Indenture ("Proposed Supplement"), a copy of which is
   attached hereto as Exhibit A. This provision does hereby constitute a
   written consent of the Holders, including all successor Holders, of the
   Series      Bonds to the execution and adoption of the Proposed
   Supplement under the provisions of Section 18.11 of the Indenture, and
   such consent is received by the Trustee as a consent for the Trustee to
   execute the Proposed Supplement in lieu of the holding of a meeting of
   Bondholders pursuant to Article Eighteen of the Indenture.
 
  At such time that the aggregate of (i) the principal amount of the Offered
Bonds outstanding and all additional Bonds hereafter issued ("Consent Bonds")
and (ii) the principal amount of all Bonds outstanding issued prior to said
date, the Holders of which have consented in writing to the adoption of the
Proposed Supplement, will equal or exceed 66 2/3 percent in principal amount of
all Bonds outstanding under the Mortgage at such time, the Trustee and the
Company will execute the Proposed Supplement causing the Twenty Eighth
Supplemental Indenture to be effective; provided, in case one or more but less
than all of the series of Bonds outstanding at such time are to be affected by
the Proposed Supplement, the Trustee may not execute the Proposed Supplement
until the Holders of 66 2/3 percent of each of the series affected have
approved the Proposed Supplement. Assuming the expected amount of Consent Bonds
are issued and certain existing Bonds are redeemed, the Company believes that
the Consent Bonds will constitute over 66 2/3 percent in principal of all
outstanding Bonds by March 1, 1998, even without any of the existing
Bondholders consenting to the Proposed Supplement. By that time, it is expected
that the Proposed Supplement will affect all outstanding issues of Bonds, and
the Company and Trustee will execute the Proposed Supplement, and the Mortgage
will be amended accordingly. If sufficient consents are received from existing
Bondholders, the Proposed Supplement will be executed and become effective at
an earlier date.
 
  Throughout this Description of the Offered Bonds, references will be made to
the Mortgage as it exists at the date of this Prospectus and how the Mortgage
will be amended by the Proposed Supplement if adopted.
 
Payment of Bonds; Transfers; Exchanges
 
  Except as may be provided in the applicable Prospectus Supplement, interest
on each Offered Bond payable on each interest payment date will be paid by
check mailed to the registered holder of such Offered Bond on the record date
related to such interest payment date; provided, interest payable at maturity
will be paid to the person to whom principal is paid.
 
                                       4
<PAGE>
 
  Principal of and premium, if any, and interest on the Offered Bonds due at
maturity will be payable upon presentation of the Bonds at the principal office
of the Trustee which has been designated by the Company as its office or agency
for such payment.
 
  The transfer of Offered Bonds will be registered, and Bonds may be exchanged
for other Bonds, upon the surrender thereof at the principal office of the
Trustee which has been designated by the Company as its office or agency for
such purposes. Unless otherwise indicated in the applicable Prospectus
Supplement, no service charge will be made for any transfer or exchange other
than any tax or other governmental charge incident thereto.
 
Withdrawal of Trust Moneys or Retirement of Bonds with Trust Moneys
 
  Under the Mortgage, cash (Trust Moneys) is deposited with the Trustee under
certain conditions, including the taking of property by governmental
authorities under the right of eminent domain, the deposit of cash for the
release of property, the deposit of cash in exchange for the issuance of new
Bonds, and the deposit of the proceeds of insurance. The Mortgage provides that
the Company, under certain conditions, may withdraw these Trust Moneys by
certifying Property Additions or by depositing previously issued Bonds.
(Sections 8.02, 8.03 and 8.04 of the Mortgage.) The Company may cause the
Trustee under certain conditions to apply Trust Moneys in payment of the
principal of Bonds upon redemption or upon tender. (Section 8.05 of the
Mortgage.) Trust Moneys from insurance proceeds may be withdrawn for the
purpose of repairing, restoring or replacing the property destroyed. (Section
8.06 of the Mortgage.) Trust Moneys may further be withdrawn for the purposes
of paying the Company's income tax obligations arising from the disposition of
properties or securities. (Section 8.07 of the Mortgage.)
 
  Under the Mortgage, the Trustee is obligated to apply Trust Moneys for the
redemption of Bonds if the Trustee holds Trust Moneys in excess of $25,000,
exclusive of monies received from the proceeds of insurance, and the amount of
Trust Moneys on deposit with the Trustee exceeded such amount at all times
during the preceding two years. The Trustee is also obligated to apply Trust
Moneys derived from the sale and release of, or the taking by government
through eminent domain or purchase by a public authority of either the entire
Trust Estate or substantially all of the business of distributing electricity
in Rapid City, South Dakota toward the redemption of Bonds. The Trustee is
obligated to apply the Trust Moneys pro rata as between the several series of
Bonds then outstanding in the ratio of the respective aggregate principal
amounts in each such series outstanding at the time. Redemption shall be made
in such manner and upon such notice as may be specified in respect of the Bonds
of each series or the Mortgage or applicable supplements thereto. (Section
8.08(a), as amended by Section 1.03 of the Fifteenth Supplemental Indenture,
and Section 8.08(b) of the Mortgage.)
 
  Section 1.15 of the Proposed Supplement would amend Section 8.08(b) of the
Mortgage to delete the provision requiring the proceeds of a public taking of
the Company's electric business at Rapid City, South Dakota to be applied
toward the redemption of Bonds.
 
Redemption of the Offered Bonds
 
  Any terms for the optional or mandatory redemption of Offered Bonds will be
set forth in the Prospectus Supplement. Except as shall otherwise be provided
in the applicable Prospectus Supplement with respect to Offered Bonds
redeemable at the option of the Holder, Offered Bonds will be redeemable only
upon notice by mail not less than 30 days prior to the date fixed for
redemption, and, if less than all the Offered Bonds of a series are to be
redeemed, the particular Offered Bonds to be redeemed will be selected by such
method as shall be provided for in the particular series, or in the absence of
any such provision, by lot as the Trustee deems fair and appropriate. (Section
10.03 of the Mortgage.)
 
Security
 
  The Offered Bonds, together with all other Bonds now or hereafter issued
under the Mortgage, will be secured by the Mortgage, which constitutes a first
mortgage lien on substantially all of the present properties
 
                                       5
<PAGE>
 
of the Company (except as stated below), subject to Permitted Encumbrances as
defined at Section 4.01E of the Mortgage. Excepted Property (defined in the
Sixteenth Granting Clause of the Mortgage) is not subject to the lien of the
Mortgage and includes all cash and securities, all contracts (other than
contracts for the purchase or exchange of electric energy or for the making of
connections for exchange of energy or service, contracts and rights for the
crossing of railroad rights-of-way, and all joint pole contracts and rights,
agreements and understandings for the use of the property, facilities and
rights of way of other public utilities), rents, incomes or profits,
electricity, materials, supplies and merchandise offered for sale in the
ordinary course of business, fuel and other consumables; and motor vehicles;
provided, the Company may cause any of such Excepted Property to become subject
to the lien of the Mortgage. The Company's 20 percent interest in the 330 MW
coal-fired Wyodak Plant near Gillette, Wyoming is not now subject to the
Mortgage. (Article One of the Eighteenth Supplemental Indenture.) However, the
Company will certify its interest in the Wyodak Plant as Property Additions for
the first series of Bonds to be issued under this Prospectus and thus subject
such interest to the lien of the Mortgage.
 
  The Proposed Supplement, if adopted, would delete the Eleventh, Twelfth and
Thirteenth Granting Clauses of the Mortgage and thereby remove from the lien of
the Indenture all contracts and agreements, all leases and all franchises.
(Section 1.01 of the Proposed Supplement.) The Proposed Supplement would also
amend the definition of Excepted Property in the Sixteenth Granting Clause of
the Mortgage to except certain additional property from the lien of the
Mortgage, including (i) all property not used in the electric utility business,
(ii) all permits, licenses, franchises and rights granted by governmental
entities, (iii) all movable equipment and parts used in connection therewith,
(iv) all office furniture and equipment, communications equipment and computer
equipment, (v) all minerals, crops and timber harvested or extracted from land
and (vi) all leasehold interests. (Section 1.03 of the Proposed Supplement.)
 
  The Trustee has a lien for its compensation, expenses and indemnity on the
Trust Estate and the proceeds thereof prior to the lien of the Bonds. (Section
15.02(d) of the Mortgage.)
 
Issuance of Additional Bonds
 
  Requirements for the issuance of additional bonds. The Mortgage provides for
no limitation on the amount of Bonds which may be issued thereunder. Additional
Bonds of any series may be issued from time to time on the basis of: (i) 60
percent of qualified Property Additions after adjustments to offset
retirements; (ii) 100 percent of previously issued Bonds retired except when
otherwise provided by the Supplemental Indenture authorizing the retired Bonds
and (iii) deposits of cash. With certain exceptions in the case of (ii) and
(iii) above, the issuance of Additional Bonds is subject to Certifiable Net
Earnings being at least equal to two times annual Interest Charges on Long-Term
Debt of the Company for a period of 12 consecutive months within the 15 months
prior to the certification. (Paragraph F of Section 4.02 as amended by Section
1.04 of the Nineteenth Supplemental Indenture.)
 
  The Proposed Supplement would increase from 60 percent to 70 percent the
principal amount of qualified Property Additions that may be used for the
issuance of additional Bonds, would modify the test period for certification of
net earnings to 12 consecutive months out of the preceding 18 months prior to
certification, and would modify the definition of Property Additions and the
formula to determine the interest coverages. (Sections 1.04, 1.07, 1.09 and
1.12 of the Proposed Supplement.)
 
  As of May 1, 1994 the Company had Unbonded Property Additions, including its
20 percent interest in the Wyodak Plant, of approximately $120,000,000 which
may be certified for the purpose of authorizing the issuance of additional
Bonds. These Unbonded Property Additions would allow the issuance of
$72,000,000 of additional Bonds under the present 60 percent requirement.
Taking into account the retroactive effect of Section 1.12 of the Proposed
Supplement explained in the following paragraph, the current Unbonded Property
Additions would allow the issuance of an additional $12,000,000 of Bonds after
the Proposed Supplement has been adopted.
 
  Section 1.12 of the Proposed Supplement would add Section 4.04 to the
Mortgage that would in effect make retroactive to May 1, 1994 the deletion of
the 15 percent maintenance requirements as deleted by
 
                                       6
<PAGE>
 
Section 1.16 of the Proposed Supplement (See Particular Covenants of the
Company--Maintenance Requirements under DESCRIPTION OF THE OFFERED BONDS) and
increase the ability to issue new Bonds based on Property Additions from 60
percent to 70 percent. After the Proposed Supplement has been adopted, the
Company will be permitted to certify for the purpose of issuing new Bonds 70
percent of (i) those Property Additions used to satisfy the 15 percent
maintenance requirements since May 1, 1994 and (ii) 14.285 percent of those
Property Additions used to issue new Bonds during the period commencing May 1,
1994 and ending on the date the Proposed Supplement is adopted.
 
  Property Additions. Under the Mortgage, Property Additions that may be
certified for the issuance of additional Bonds include only Electric Properties
which are used in the generation, transmission, distribution and sale of
electricity in the Electric Business located in the State of South Dakota or
states contiguous to South Dakota, but do not include any of the Excepted
Property (See Security under DESCRIPTION OF OFFERED BONDS), any properties
other than Electric Properties, any going concern value, goodwill or franchises
or governmental permits granted separate and distinct from the property
operated thereunder, any office supplies or office equipment, or any Additions
in the process of construction other than those that are actually constructed
or erected. (Section 4.01 of the Mortgage.) The Proposed Supplement would
change this definition to allow the Company to use all of its real and personal
property wherever located, including Excepted Property that the Company elects
to be included under the lien of the Mortgage, to be certified as Property
Additions for the purpose of issuing additional Bonds. The Proposed Supplement
would also broaden the use of construction work in progress that may be used as
Property Additions to include all construction work in progress as recorded on
the books of account of the Company under generally accepted accounting
principles. (Section 1.04 of the Proposed Supplement.)
 
  Earnings-to-Interest Coverage Requirements. To issue new Bonds through the
certification of Property Additions and as a condition to issue new Bonds in
exchange for retired Bonds with a lower interest rate than the new Bonds, the
Company's Certifiable Net Earnings must be equal to two times the Annual
Interest Charges on Long-Term Debt during a period of 12 consecutive months out
of the 15 months prior to the date of the application for the new Bonds.
(Subparagraph (3) of Paragraph F of Section 4.02 of the Mortgage as amended by
Section 1.04 of the Nineteenth Supplemental Indenture.) Certifiable Net
Earnings are determined by deducting from the total of the gross operating
revenues of the Company for the 12-month period all operating expenses and
provisions for depreciation and depletion, but excluding income taxes and
interest included in Interest Charges on Long-Term Debt. The depreciation,
obsolescence, depletion and property renewals and placements must be an amount
not less than the greater of the amount actually deducted on the books of the
Company in respect thereof or an amount equal to the excess of 15 percent of
Gross Utility Operating Revenues, less the cost of electric energy, over actual
expenditures for maintenance and repairs during the 12-month period. (Paragraph
O of Section 4.01 as last amended by Section 1.03 of Article One of the
Nineteenth Supplemental Indenture.)
 
  Gross Operating Revenues include revenues from the operation of utility
property and Net Non-Operating Income includes other net income, including,
among other things, interest and designated income including dividends from
Subsidiaries and allowance for funds used during construction and other
miscellaneous non-operating income, but excluding profits or losses from the
sale of capital assets or securities. Interest Charges on Long-Term Debt
include interest on all outstanding Bonds under the Mortgage, the Bonds for
which the application is made, interest on any indebtedness secured by prior
lien to the Mortgage and interest from all other Funded Indebtedness.
(Paragraph F of Section 4.02 of the Mortgage as amended by Section 1.04 of the
Nineteenth Supplemental Indenture.) Funded Indebtedness generally includes all
indebtedness, whether secured or unsecured, having a final maturity more than
one year after the date of the creation thereof except that under certain
circumstances a Financing Lease Obligation would not be included. (Section 4.04
of the Seventeenth Supplemental Indenture as amended by Section 2.01 of the
Eighteenth Supplemental Indenture.)
 
  The Proposed Supplement would retain the two times coverage ratio but would
adopt three substantive changes to the formula. Certifiable Net Earnings would
be determined without deducting depreciation, amortization, all interest and
all nonrecurring charges from earnings. Interest Charges on Long-Term Debt
 
                                       7
<PAGE>
 
would become Interest Charges on Bonds and Prior Lien Debt so that Certifiable
Net Earnings would be required to be two times the aggregate amount of annual
interest charges on outstanding Bonds issued under the Mortgage, on Prior Lien
debt and on the new Bonds for which application is being made in order to meet
the coverage test after the adoption of the Proposed Supplement. The Proposed
Supplement also allows the 12-month test period to be chosen from the 18 months
prior to the date of the application for the new Bonds rather than 15 months.
(Section 1.09 of the Proposed Supplement.)
 
Release of Property from Mortgage
 
  The Mortgage provides that the Company may from time to time sell or dispose
of property subject to the Mortgage which shall no longer be useful, necessary,
profitable or advantageous in the judicious management and maintenance of the
Trust Estate or in the conduct of the business of the Company or which is taken
or threatened to be taken by governmental authorities and to cause such
property to be released from the lien of the Mortgage. To accomplish the
release of any property above the value of $5,000, the Company is required to
deposit cash or purchase money obligations (any such purchase money obligations
may not exceed 70 percent of the Fair Value) received in exchange for the
property to be released or Property Additions acquired by the Company within 60
days prior to the date of the application for the release in an amount equal to
the Fair Value of the property being released. (Section 7.02 of the Mortgage.)
 
  Section 1.13 of the Proposed Supplement is an extensive revision of Section
7.02 of the Mortgage. After its adoption, the Company will no longer have to
certify any reason for requesting property to be released from the lien of the
Mortgage. The Proposed Supplement would provide for three alternative ways for
the Company to obtain the release of property from the lien of the Mortgage.
Each way works independently of the other, and each is conditioned upon there
being no Event of Default.
 
  First, the Proposed Supplement would permit property to be released from the
Mortgage if the Fair Value (as defined by the Proposed Supplement) of the
property to be released is less than 1 percent of the aggregate principal
amount of Bonds outstanding and all property released in any 12-month period is
less than 3 percent of the principal amount of Bonds outstanding. (amended
Section 7.02C of the Mortgage as set forth in Section 1.13 of the Proposed
Supplement.)
 
  Second, any property may be released from the Mortgage under the Proposed
Supplement as long as the Fair Value of all of the Trust Estate remaining
equals or exceeds twenty-fourteenths ( 20/14) of the principal amount of Bonds
outstanding. (amended Section 7.02B of the Mortgage as set forth in Section
1.13 of the Proposed Supplement.)
 
  Third, amended Section 7.02D as set forth in Section 1.13 of the Proposed
Supplement would substantially retain the present method to release property
from the Mortgage by posting Property Additions, cash or purchase money
obligations equal to the Fair Value of the property to be released; however,
the Proposed Supplement would remove the requirement that the Property
Additions must have been acquired 60 days prior to the date of the application
for the release. Accordingly, any unbonded Property Additions could be used for
the release of property from the Mortgage.
 
  "Fair Value" in the Proposed Supplement is defined to allow the engineer
certifying Fair Value for the purpose of releasing property from the Mortgage
to determine such value without physical inspection by use of accounting and
engineering records and other data maintained by or available to the Company,
and the engineer must opine that the release will not materially adversely
affect the Electric Business and will not impair the security under the
Mortgage. (amended Section 7.02A and 7.02B(2) of the Mortgage as set forth in
Section 1.13 of the Proposed Supplement.)
 
 
                                       8
<PAGE>
 
Particular Covenants of the Company
 
  The Mortgage contains certain covenants, agreements and warranties of the
Company, generally described as follows:
 
  General. Included in the covenants, agreements and warranties of the Company
in the Mortgage are the agreement to punctually pay the principal and interest
on the Bonds and the agreement not to extend the time for payment of any
interest on the Bonds. The Company also covenants that it has good title to the
property described in the Granting Clauses except Permitted Encumbrances, that
the Company will pay all taxes and assessments, that the Company will carry on
and conduct the business and keep the property in repair and maintained in good
working order and condition and replace worn-out or injured property, that the
Company will not permit any increase of any Prior Lien Obligation or default
therein, that the Company will keep the Mortgage properly recorded and the lien
perfected, that the Company will deposit all cash subject to being withdrawn as
required by the Indenture, that the Company will keep records of the accounts
of the Bonds outstanding, that the Company will keep books and records, and
that the Company will file with the Commission and deliver to the Trustee
certain reports and that the Company will certify compliance with the Mortgage
annually. (Sections 9.01 through 9.05, 9.07 through 9.09, 9.11 through 9.14,
9.17 through 9.19 and 9.22 of the Mortgage.) The covenants, agreements and
warranties referenced in this paragraph would not be modified by the Proposed
Supplement.
 
  Maintenance Requirements. Section 9.06 of the Mortgage requires the Company
to spend each year 15 percent of its Gross Operating Revenues (not including
the purchase of electric energy) on maintenance and repair of its physical
property. To the extent the Company does not spend the 15 percent, the Company
must deposit cash or certify Property Additions. Section 1.16 of the Proposed
Supplement would delete this Section 9.06, and if the Proposed Supplement is
adopted, the Company will no longer be required to certify the amount expended
for repair and maintenance nor post cash or certified Property Additions for
any deficiency. Provisions will remain in the Mortgage obligating the Company
to maintain its business and to keep its property in repair and good working
order and condition. (Section 9.13 and the first paragraph of Section 9.05 of
the Mortgage as will be amended by Section 1.02 of the Proposed Supplement.)
 
  Subsidiaries Prohibited from Engaging in Electric Business. Section 9.20 of
the Mortgage prevents the Company from owning a Subsidiary (where a company
owns over 50 percent of the voting stock) that is engaged in the business of
generating, transmitting, distributing or selling electricity. Section 1.20 of
the Proposed Supplement would remove this restriction, thereby allowing the
Company to establish or acquire a Subsidiary that is engaged in the Electric
Business.
 
  Restrictions on Investments in Affiliates and Subsidiaries. Section 9.20 of
the Mortgage prohibits the Company from making any loans or advances to any
Affiliate (an entity not a Subsidiary but controlled by the Company) of the
Company. Section 9.21 as last amended by Section 1.02 of the Twenty Fourth
Supplemental Indenture prohibits the Company from making any investment in a
Subsidiary in excess of 20 percent of the Company's book equity. Sections 1.20
and 1.21 of the Proposed Supplement would delete Sections 9.20 and 9.21 of the
Mortgage and thereby remove these covenants and restrictions.
 
  Dividend Restrictions. Section 9.16 of the Mortgage as amended by Section
3.06 of the Third Supplemental Indenture, Section 1.06 of the Fifteenth
Supplemental Indenture, Section 1.02 of the Twenty Second Supplemental
Indenture and Section 1.01 of the Twenty Fourth Supplemental Indenture limit
the amount of dividends the Company may declare to the amount that (i) the sum
of $175,000, plus the Net Earnings of the Company Available for Dividends
accrued subsequent to October 31, 1948 plus the cash proceeds from the sale of
stock subsequent to April 30, 1949 exceeds (ii) the sum of the aggregate of all
Stock Payments declared subsequent to October 31, 1948, plus the aggregate of
all cash dividends and payments to any sinking funds made subsequent to October
31, 1948 plus the aggregate of all investments or advances by the Company to
any Subsidiary. The current amount of retained earnings of the Company
available for dividend distributions as of March 31, 1994 is $111,167,000.
 
 
 
                                       9
<PAGE>
 
  The Proposed Supplement would delete the current dividend restriction and
substitute a provision that would permit dividends to be paid only out of
retained earnings and would prohibit payment of dividends if the Company is
insolvent or would become insolvent if the dividend were paid. (Section 1.19 of
Proposed Supplement.)
 
  Insured Loss Retention. The third sentence of Section 9.10 of the Mortgage as
previously amended by Section 1.04 of the Fifteenth Supplemental Indenture
requires the Company to deposit with the Trustee the proceeds of any insurance
for casualty losses that exceed $1,000,000. The Proposed Supplement at Section
1.17 would amend this provision and increase the $1,000,000 to $5,000,000.
 
  Limits on Prior Liens. Subject to minor exceptions, Section 9.15 of the
Mortgage prevents the Company from acquiring any property subject to a lien
prior to the Mortgage (Prior Liens) allowing only purchase price liens not
exceeding 70 percent of the Cost of such property or the Fair Value thereof at
the time of acquisition, whichever is less. The provision further limits the
amount of all Prior Lien Obligations to an aggregate amount not exceeding 15
percent of outstanding Bonds issued under the Mortgage. Section 1.18 of the
Proposed Supplement would delete Section 9.15, and thereby remove these
restrictions.
 
Modification
 
  The rights of Bondholders may be modified with the consent of holders of 66
2/3 percent of the Bonds, or, if less than all series of Bonds are adversely
affected, the consent of the holders of 66 2/3 percent of the Bonds of each of
the outstanding series which are adversely affected by the modification. No
modification of the term or alteration shall postpone due dates for payment of
interest or principal, reduce the principal or the rate of interest payable,
reduce the percentage of the principal amount of Bonds required for any
modification or alteration of the Mortgage or modify, without the written
consent of the Trustee, the rights, duties or immunities of the Trustee. The
Proposed Supplement would not affect this provision. (Sections 18.07 and 18.11
of the Mortgage.)
 
  Without any consent of the holders of the Offered Bonds and other
Bondholders, the Company and the Trustee may modify the Mortgage in the
following respects:
 
  (a) To correct descriptions of property and to mortgage additional
properties;
 
  (b) To add to the conditions, limitations and restrictions on the issuance of
Bonds;
 
  (c) To add to the covenants and agreements of the Company or to surrender any
      rights or powers conferred upon the Company;
 
  (d) To provide a sinking, amortization, improvement or other analogous fund
      for the benefit of any of the Bonds;
 
  (e) To provide the terms and conditions of the exchange of Bonds;
 
  (f) To provide that the principal of Bonds of any series may be converted at
      the option of the holders into capital stock or other securities;
 
  (g) To modify the Mortgage in any respect; provided that the modification
      shall become effective only when there are no Bonds outstanding delivered
      prior to the execution of the supplemental indenture providing the
      modification; and
 
  (h) For any purpose not inconsistent with the terms of the Mortgage and which
      shall not impair the security of the same or for the purpose of curing
      any ambiguity or of curing, correcting, or supplementing any defective or
      inconsistent provisions contained therein or in any Supplemental
      Indenture. (Section 17.01 of the Mortgage.)
 
  The Proposed Supplement would, in addition to the above powers, authorize the
Trustee to (i) provide for the procedures to permit the Company to utilize, at
its option, a noncertificated system of registration for
 
                                       10
<PAGE>
 
all or any series of the Bonds and (ii) enter into a restatement of the
Indenture without material modifications and including all amendments contained
in supplements that remain in effect, with authority to reorganize materials,
renumber and letter, include reference headings and remove language no longer
applicable and clarify any ambiguities in the Indenture as amended without any
approval of the Offered Bonds or other Bondholders. (Section 1.23 of the
Proposed Supplement.)
 
Events of Default and Notice
 
  Events of Default are defined in the Mortgage as: default in payment of
principal; default for 30 days in payment of interest; default for 30 days in
violating the covenant on dividend restrictions; default for 60 days after
notice from the Trustee or 10 percent of the Bondholders in the observance of
any other covenant or condition; and certain events in bankruptcy, insolvency
or reorganization.
 
  The Trustee or the holders of 25 percent of the Bonds may declare the
principal and interest due and payable on an Event of Default. (Section 12.01
of the Mortgage.) No Holder of Bonds may enforce the lien of the Mortgage
unless the holders of a majority in amount of the Bonds then outstanding have
requested the Trustee to act and offered it indemnity satisfactory to the
Trustee against the costs, expenses and liabilities to be incurred thereby and
the Trustee shall have failed to act. (Sections 12.04 and 15.02 of the
Mortgage.)
 
Evidence of Compliance with Mortgage Provisions
 
  Compliance with the Mortgage provisions is evidenced by written statements of
Company officers or persons selected or paid by the Company. In certain cases,
opinions of counsel and certification of an engineer, accountant, appraiser or
other expert (who in some cases must be independent) must be furnished. The
Company must give the Trustee an annual statement as to whether or not the
Company has fulfilled its obligations under the Mortgage throughout the
preceding calendar year.
 
Defeasance
 
  The Trustee shall satisfy and discharge the Mortgage upon the Company
furnishing the Trustee cash in trust at or before maturity sufficient to
discharge the entire indebtedness on the Bonds outstanding or to redeem the
Bonds, except that in lieu of cash the Company may deposit the Bonds
outstanding and certain other resolutions, certificates and opinions. (Article
Sixteen of the Mortgage.)
 
                                    EXPERTS
 
  The audited financial statements and supplemental financial statement
schedules included in the 1993 Annual Report of the Company on Form 10-K,
incorporated herein by reference, have been audited by Arthur Andersen & Co.,
Independent Public Accountants, as stated in their reports. The audited
financial statements and financial statement schedules referred to above have
been incorporated by reference herein in reliance upon the authority of said
firm as experts in giving said reports. Future financial statements of the
Company and the reports thereon of Arthur Andersen & Co. also will be
incorporated by reference in this prospectus in reliance upon the authority of
that firm as expert in giving those reports to the extent said firm has audited
those financial statements and consented to the use of their reports thereon.
 
                                 LEGAL OPINIONS
 
  The legality of the securities offered hereby will be passed upon for the
Company by Morrill Brown & Thomas, 625 Ninth Street, Rapid City, South Dakota
57709, general counsel of the Company, and for any underwriter, dealer or agent
by Brown & Wood, One World Trade Center, New York, New York 10048-0557. All
matters pertaining to local law will be passed upon by Morrill Brown & Thomas.
As of May 1, 1994 members and associates of the firm of Morrill Brown & Thomas
owned approximately 8,100 shares of the common stock of the Company.
 
                              PLAN OF DISTRIBUTION
 
  The Company may sell the Offered Bonds in any of three ways: (i) through
underwriters or dealers; (ii) directly to a limited number of purchasers or to
a single purchaser; or (iii) through agents. The Prospectus
 
                                       11
<PAGE>
 
Supplement with respect to the Offered Bonds sets forth the terms of the
offering of the Offered bonds, including the name or names of any underwriters,
dealers or agents, the purchase price of such Offered Bonds and the proceeds to
the Company from such sale, any underwriting discounts and other items
constituting underwriters' compensation, any initial public offering price and
any discounts or concessions allowed or reallowed or paid to dealers. Any
initial public offering price and any discounts or concessions allowed or
reallowed or paid to dealers may be changed from time to time.
 
  If underwriters are used in the sale, the Offered Bonds will be acquired by
the underwriters for their own account and may be resold from time to time in
one or more transactions, including negotiated transactions, at a fixed public
offering price or at varying prices determined at the time of the sale. The
underwriter or underwriters with respect to a particular underwritten offering
of Offered bonds are named in the Prospectus Supplement relating to such
offering and, if an underwriting syndicate is used, the managing underwriter or
underwriters are set forth on the cover page of such Prospectus Supplement.
Unless otherwise set forth in the Prospectus Supplement, the obligations of the
underwriters to purchase the Offered Bonds will be subject to certain
conditions precedent, and the underwriters will be obligated to purchase all
such Offered Bonds if any are purchased.
 
  Offered Bonds may be sold directly by the Company or through agents
designated by the Company from time to time. The Prospectus Supplement sets
forth the name of any agent involved in the offer or sale of the Offered Bonds
in respect of which the Prospectus Supplement is delivered as well as any
commissions payable by the Company to such agent. Unless otherwise indicated in
the Prospectus Supplement, any such agent will be acting on a best efforts
basis for the period of its appointment.
 
  If so indicated in the Prospectus Supplement, the Company will authorize
agents, underwriters or dealers to solicit offers by certain specified
institutions to purchase Offered Bonds from the Company at the public offering
price set forth in the Prospectus Supplement pursuant to delayed delivery
contracts providing for payment and delivery on a specified date in the future.
Such contracts will be subject to those conditions set forth in the Prospectus
Supplement, and the Prospectus Supplement will set forth the commission payable
for solicitation of such contracts.
 
  Subject to certain conditions, the Company may agree to indemnify the several
underwriters or agents and their controlling persons against certain
liabilities, including liabilities under the 1933 Act arising out of or based
upon, among other things, any untrue statement or alleged untrue statement of a
material fact contained in the registration statement, this Prospectus, a
Prospectus Supplement or the incorporated documents or the omission or alleged
omission to state therein a material fact required to be stated therein or
necessary to make the statements therein, in the light of the circumstances
under which they were made, not misleading. See the Prospectus Supplement.
 
  No dealer, salesman or other person has been authorized to give any
information or to make any representations other than those contained in this
Prospectus and any Prospectus Supplement in connection with the offer made by
this Prospectus or any Prospectus Supplement and, if given or made, such
information or representations must not be relied upon as having been
authorized by the Company or any other person, underwriter, dealer or agent.
Neither the delivery of this Prospectus or any Prospectus Supplement nor any
sale made hereunder shall under any circumstances create an implication that
there has been no change in the affairs of the Company since the date hereof or
thereof. This Prospectus or any Prospectus Supplement does not constitute an
offer or solicitation by anyone in any jurisdiction in which such offer or
solicitation is not authorized or in which the person making such offer or
solicitation is not qualified to do so or to anyone to whom it is unlawful to
make such offer or solicitation.
 
                                       12
<PAGE>
 
                                    PART II
 
                     INFORMATION NOT REQUIRED IN PROSPECTUS
 
Item 14. Other Expenses of Issuance and Distribution
 
  Set forth below is an estimate of the approximate amount of fees and expenses
payable by the Registrant (other than underwriting discounts and commissions)
in connection with the issuance of the Shares:
 
<TABLE>
<CAPTION>
   Description                                                           Amount
   -----------                                                          --------
   <S>                                                                  <C>
   Registration fee under the Securities Act of 1933................... $ 34,483
   Accounting Services.................................................   30,000
   Attorneys' fees.....................................................   50,000
   Blue sky fees.......................................................    5,000
   Fees of ratings agencies............................................   51,000
   Printing............................................................   20,000
   Trustees' fees......................................................   15,000
   Other miscellaneous costs...........................................   10,517
                                                                        --------
   Total............................................................... $216,000
</TABLE>
- --------
All items are estimated except the first.
 
Item 15. Indemnification of Directors and Officers
 
  Sections 47-2-58.1 to 47-2-58.7 of the South Dakota Codified Laws permit
indemnification of officers and directors of domestic or foreign corporations
under certain circumstances and subject to certain limitations. Article V of
the Bylaws of the Company and separate indemnification contracts entered into
between the Company and each of its directors and officers authorize
indemnification of the Company's directors and officers consistent with the
provisions of South Dakota laws.
 
  The Company has obtained insurance policies indemnifying the Company and the
Company's directors and officers against certain civil liabilities and related
expenses.
 
Item 16. Exhibits
 
<TABLE>
 <C>      <S>                                                        <C>              <C>
  1.01    Proposed form of Underwriting Agreement
  4.04    Form of First Mortgage Bond
<CAPTION>
                                                                         File or
                                                                     Registration No. Exhibit
                                                                     ---------------- -------
 <C>      <S>                                                        <C>              <C>
 *4.05(a) Indenture of Mortgage and Deed of Trust, dated September       Form A-2        B
          1, 1941                                                        2-4832
 *4.05(b) First Supplemental Indenture, dated July 15, 1945              Form S-1       7-B
                                                                         2-6576
 *4.05(c) Second Supplemental Indenture, dated January 15, 1948          Form S-1       7-C
                                                                         2-7695
 *4.05(d) Third Supplemental Indenture, dated January 15, 1949           Form S-1       7-D
                                                                         2-8157
  4.05(e) Fourth Supplemental Indenture, dated March 1, 1950
 *4.05(f) Fifth Supplemental Indenture, dated March 1, 1952              Form S-1       4-I
                                                                         2-9433
 *4.05(g) Sixth Supplemental Indenture, dated July 1, 1956               Form S-1       4-H
                                                                         2-13140
 *4.05(h) Seventh Supplemental Indenture, dated May 1, 1957              Form S-1       4-I
                                                                         2-14829
</TABLE>
 
                                      II-1
<PAGE>
 
<TABLE>
<CAPTION>
                                                                        File or
                                                                    Registration No. Exhibit
                                                                    ---------------- -------
<S>        <C>                                                      <C>              <C>
*4.05(i)   Eighth Supplemental Indenture, dated May 1, 1959         Form S-1           4-J
                                                                    2-16756
*4.05(j)   Ninth Supplemental Indenture, dated April 1, 1960        Form S-1           4-K
                                                                    2-16756
*4.05(k)   Tenth Supplemental Indenture, dated August 1, 1960       Form S-1           4-L
                                                                    2-21024
*4.05(l)   Eleventh Supplemental Indenture, dated June 1, 1961      Form S-1           4-M
                                                                    2-21024
*4.05(m)   Twelfth Supplemental Indenture, dated October 1, 1962    Form S-1           4-N
                                                                    2-21024
*4.05(n)   Thirteenth Supplemental Indenture, dated May 1, 1963     Form S-7          2(q)
                                                                    2-57661
*4.05(o)   Fourteenth Supplemental Indenture, dated June 1, 1969    Form S-7          2(r)
                                                                    2-57661
*4.05(p)   Fifteenth Supplemental Indenture, dated June 15, 1974    Form S-7          2(s)
                                                                    2-57661
*4.05(q)   Sixteenth Supplemental Indenture, dated August 1, 1974   Form S-7          2(t)
                                                                    2-57661
*4.05(r)   Seventeenth Supplemental Indenture, dated July 15, 1975  Form S-7          2(u)
                                                                    2-57661
*4.05(s)   Eighteenth Supplemental Indenture, dated May 1, 1976     Form S-7          2(v)
                                                                    2-57661
 4.05(t)   Nineteenth Supplemental Indenture, dated February 15,
           1977
 4.05(u)   Twentieth Supplemental Indenture, dated April 1, 1977
 4.05(v)   Twenty First Supplemental Indenture, dated June 1, 1977
*4.05(w)   Twenty Second Supplemental Indenture, dated July 14,     Form S-3          4(b)
           1982                                                     2-81643
 4.05(x)   Twenty Third Supplemental Indenture, dated September 1,
           1986
 4.05(y)   Twenty Fourth Supplemental Indenture, dated April 13,
           1987
 4.05(z)   Twenty Fifth Supplemental Indenture, dated June 15, 1988
*4.05(aa)  Twenty Sixth Supplemental Indenture, dated May 15, 1991  Amendment #1 to   4(d)
                                                                    Form S-8
                                                                    33-15868
*4.05(ab)  Twenty Seventh Supplemental Indenture, dated June 1,     Amendment #1 to   4(e)
           1991                                                     Form S-8
                                                                    33-15868
 4.06      Form of Proposed Twenty Eighth Supplemental Indenture
 5.01      Opinion of Morrill Brown & Thomas as to legality of Offered Bonds
12.01      Computation of Ratio of Earnings to Fixed Charges
23.01      Consent of Independent Public Accountants
23.02      Consent of Legal Counsel (included in Exhibit 5.01)
25.01      Form T-1 Statement of Eligibility and Qualification under the
           Trust Indenture Act of 1939 of Chemical Bank
</TABLE>
 
* Exhibits incorporated by reference.
 
                                      II-2
<PAGE>
 
Item 17. Undertakings
 
  The undersigned registrant hereby undertakes:
 
  (1) To file, during any period in which offers or sales are being made, a
post-effective amendment to this Registration Statement: (i) to include any
prospectus required by Section 10(a)(3) of the Securities Act of 1933; (ii) to
reflect in the Prospectus any facts or events arising after the effective date
of the Registration Statement (or the most recent post-effective amendment
thereof) which, individually or in the aggregate, represent a fundamental
change in the information set forth in the Registration Statement; or (iii) to
include any material information with respect to the plan of distribution not
previously disclosed in the Registration Statement or any material change to
such information in the Registration Statement; provided, however, that the
registrant need not file a post-effective amendment to include the information
required to be included by subsection (i) or (ii) if such information is
contained in periodic reports filed by the registrant pursuant to Section 13 or
Section 15(d) of the Exchange Act, which are incorporated by reference in the
Registration Statement.
 
  (2) That, for the purpose of determining any liability under the Securities
Act of 1933, each such post-effective amendment shall be deemed to be a new
registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.
 
  (3) To remove from registration by means of a post-effective amendment any of
the securities being registered which remain unsold at the termination of the
offering.
 
  (4) That, for purposes of determining any liability under the Securities Act
of 1933, each filing of the registrant's annual report pursuant to Section
13(a) or Section 15(d) of the Exchange Act that is incorporated by reference in
this Registration Statement shall be deemed to be a new registration statement
relating to the securities offered herein, and the offering of such securities
at that time shall be deemed to be the initial bona fide offering thereof.
 
  Insofar as indemnification for liabilities arising under the Securities Act
of 1933 may be permitted to directors, officers and controlling persons of the
registrant pursuant to the foregoing provisions described under Item 15, or
otherwise, the registrant has been advised that in the opinion of the
Securities and Exchange Commission such indemnification is against public
policy as expressed in the Securities Act of 1933 and is, therefore,
unenforceable. In the event that a claim for indemnification against such
liabilities (other than the payment by the registrant of expenses incurred or
paid by a director, officer or controlling person of the registrant in the
successfuldefense of any action, suit or proceeding) is asserted by such
director, officer or controlling person in connection with the securities being
registered, the registrant will, unless in the opinion of its counsel the
matter has been settled by controlling precedent, submit to a court of
appropriate jurisdiction the question whether such indemnification by it is
against public policy as expressed in the Securities Act of 1933 and will be
governed by the final adjudication of such issue.
 
                                      II-3
<PAGE>
 
                                   SIGNATURES
 
  Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-3 and has duly caused this Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Rapid City, State of South Dakota, on the 28th day
of June, 1994.
 
                                              Black Hills Corporation
 
                                              By /s/ Dale E. Clement
                                                Dale E. Clement--Senior Vice
                                                President--Finance
 
  Pursuant to the requirements of the Securities Act of 1933, this Registration
Statement has been signed by the following persons in the capacities and on the
dates indicated.
 
         Name                    Position                        Date
/s/ Daniel P. Landguth      Director and Principal
Daniel P. Landguth             Executive Officer
(Chairman, President,                                        June 28, 1994
and Chief Executive)
 
/s/ Dale E. Clement         Director and Principal
Dale E. Clement                Financial Officer
(Senior Vice                                                 June 28, 1994
President--Finance)
 
/s/ Gary R. Fish            Principal Accounting             June 28, 1994
Gary R. Fish                        Officer
(Controller)
 
/s/ Glenn C. Barber                Director                  June 28, 1994
Glenn C. Barber
 
/s/ Bruce B. Brundage              Director                  June 28, 1994
Bruce B. Brundage
 
/s/ Michael B. Enzi                Director                  June 28, 1994
Michael B. Enzi
 
/s/ John R. Howard                 Director                  June 28, 1994
John R. Howard
 
/s/ Everett E. Hoyt          Director and Officer            June 28, 1994
Everett E. Hoyt
(President and Chief
Operating Officer of
Black Hills Power)
 
/s/ Kay S. Jorgensen               Director                  June 28, 1994
Kay S. Jorgensen
 
/s/ Charles T. Undlin              Director                  June 28, 1994
Charles T. Undlin
 
                                      II-4
<PAGE>
 
                               INDEX TO EXHIBITS
 
<TABLE>
<CAPTION>
                                                      File or
 Exhibit               Description                Registration No. Exhibit Page
 -------               -----------                ---------------- ------- ----
 <C>      <S>                                     <C>              <C>     <C>
  1.01    Proposed form of Underwriting
          Agreement
  4.04    Form of First Mortgage Bond
 *4.05(a) Indenture of Mortgage and Deed of           Form A-2        B     *
          Trust, dated September 1, 1941               2-4832
 *4.05(b) First Supplemental Indenture, dated         Form S-1       7-B    *
          July 15, 1945                                2-6576
 *4.05(c) Second Supplemental Indenture, dated        Form S-1       7-C    *
          January 15, 1948                             2-7695
 *4.05(d) Third Supplemental Indenture, dated         Form S-1       7-D    *
          January 15, 1949                             2-8157
  4.05(e) Fourth Supplemental Indenture, dated
          March 1, 1950
 *4.05(f) Fifth Supplemental Indenture, dated         Form S-1       4-I    *
          March 1, 1952                                2-9433
 *4.05(g) Sixth Supplemental Indenture, dated         Form S-1       4-H    *
          July 1, 1956                                2-13140
 *4.05(h) Seventh Supplemental Indenture, dated       Form S-1       4-I    *
          May 1, 1957                                 2-14829
 *4.05(i) Eighth Supplemental Indenture, dated        Form S-1       4-J    *
          May 1, 1959                                 2-16756
 *4.05(j) Ninth Supplemental Indenture, dated         Form S-1       4-K    *
          April 1, 1960                               2-16756
 *4.05(k) Tenth Supplemental Indenture, dated         Form S-1       4-L    *
          August 1, 1960                              2-21024
 *4.05(l) Eleventh Supplemental Indenture,            Form S-1       4-M    *
          dated June 1, 1961                          2-21024
 *4.05(m) Twelfth Supplemental Indenture, dated       Form S-1       4-N    *
          October 1, 1962                             2-21024
 *4.05(n) Thirteenth Supplemental Indenture,          Form S-7      2(q)    *
          dated May 1, 1963                           2-57661
 *4.05(o) Fourteenth Supplemental Indenture,          Form S-7      2(r)    *
          dated June 1, 1969                          2-57661
 *4.05(p) Fifteenth Supplemental Indenture,           Form S-7      2(s)    *
          dated June 15, 1974                         2-57661
 *4.05(q) Sixteenth Supplemental Indenture,           Form S-7      2(t)    *
          dated August 1, 1974                        2-57661
 *4.05(r) Seventeenth Supplemental Indenture,         Form S-7      2(u)    *
          dated July 15, 1975                         2-57661
 *4.05(s) Eighteenth Supplemental Indenture,          Form S-7      2(v)    *
          dated May 1, 1976                           2-57661
  4.05(t) Nineteenth Supplemental Indenture,
          dated February 15, 1977
  4.05(u) Twentieth Supplemental Indenture,
          dated April 1, 1977
</TABLE>
<PAGE>
 
<TABLE>
<CAPTION>
                                                      File or
  Exhibit               Description               Registration No. Exhibit Page
  -------               -----------               ---------------- ------- ----
 <C>       <S>                                    <C>              <C>     <C>
  4.05(v)  Twenty First Supplemental Indenture,
           dated June 1, 1977
 *4.05(w)  Twenty Second Supplemental                 Form S-3      4(b)    *
           Indenture, dated July 14, 1982             2-81643
  4.05(x)  Twenty Third Supplemental Indenture,
           dated September 1, 1986
  4.05(y)  Twenty Fourth Supplemental
           Indenture, dated April 13, 1987
  4.05(z)  Twenty Fifth Supplemental Indenture,
           dated June 15, 1988
 *4.05(aa) Twenty Sixth Supplemental Indenture,     Amendment #1    4(d)    *
           dated May 15, 1991                       to Form S-8
                                                      33-15868
 *4.05(ab) Twenty Seventh Supplemental              Amendment #1    4(e)    *
           Indenture, dated June 1, 1991            to Form S-8
                                                      33-15868
  4.06     Form of Proposed Twenty Eighth Supplemental Indenture
  5.01     Opinion of Morrill Brown & Thomas as to legality of Offered Bonds
 12.01     Computation of Ratio of Earnings to Fixed Charges
 23.01     Consent of Independent Public Accountants
 23.02     Consent of Legal Counsel (included in Exhibit 5.01)
 25.01     Form T-1 Statement of Eligibility and Qualification under the
           Trust Indenture Act of 1939 of Chemical Bank
</TABLE>
 
*Indicates incorporation by reference

<PAGE>
 
                                                                    EXHIBIT 1.01
                                                                    ------------



                            BLACK HILLS CORPORATION

                              First Mortgage Bonds


                             UNDERWRITING AGREEMENT
                             ----------------------


                                                           ____________, 1994


MERRILL LYNCH & CO.
Merrill Lynch, Pierce, Fenner & Smith
            Incorporated
PAINEWEBBER INCORPORATED
c/o Merrill Lynch & Co.
    Merrill Lynch, Pierce, Fenner & Smith
                Incorporated
    North Tower, World Financial Center
    New York, New York 10281

Dear Sirs:

     Black Hills Corporation, a South Dakota corporation (the "Company"),
proposes to issue and sell up to $100,000,000 aggregate principal amount of its
First Mortgage Bonds (the "Securities") in one or more offerings on terms
determined at the time of sale.  The Securities will be issued under an
Indenture of Mortgage and Deed of Trust dated as of September 1, 1941, as
amended and supplemented by twenty seven supplemental indentures (together, the
"Indenture"), between the Company and Chemical Bank as trustee and success or by
merger to the original and succeeding trustees (the "Trustee").  Each issue of
Securities may vary as to aggregate principal amount, maturity, date, interest
rate or rates and timing of payments thereof, any redemption or sinking fund
requirements, and any other variable terms as the applicable supplemental
indenture (the "Supplemental Indenture") contemplates may be set forth in the
Securities issued from time to time.  As used herein, "you" and "your", unless
the context otherwise requires, shall mean such of the parties, if any, to whom
this Agreement is addressed as are named in the applicable Terms Agreement or
any such additional parties as may be specifically named in a Terms Agreement as
Managing Underwriters with respect to Securities purchased pursuant to such
Terms Agreement.

<PAGE>
 
     Offerings of Securities may be made through one or more of you or through
an underwriting syndicate managed by one or more of you.  If the Company
determines to make an offering of Securities through one or more of you or
through an underwriting syndicate managed by one or more of you, the Company
will enter into an agreement (the "Terms Agreement") providing for the sale of
such Securities (the "Offered Securities") to, and the purchase and offering
thereof by, one or more of you and such other underwriters, if any, selected by
you as have authorized you to enter into such Terms Agreement on their behalf
(the "Underwriters", which term shall include you whether acting alone in the
sale of securities or as members of an underwriting syndicate).  The Terms
Agreement relating to the Offered Securities shall specify the principal amount
of Offered Securities to be issued and their terms not otherwise specified in
the related Indenture, the names of the Underwriters participating in such
offering (subject to substitution as provided in Section 10 hereof), the
principal amount of Securities which each such Underwriter severally agrees to
purchase, the names of such of you or such other Underwriters acting as co-
managers, if any, in connection with such offering, the price at which the
Offered Securities are to be purchased by the Underwriters from the Company, the
initial public offering price and the time and place of delivery and payment.
The Terms Agreement, which shall be substantially in the form of Exhibit A
hereto, may take the form of an exchange of any standard form of written
telecommunication between you and the Company.  Each offering of Securities
through one or more of you or through an underwriting syndicate managed by one
or more of you will be governed by this Agreement, as supplemented by the
applicable Terms Agreement, and this Agreement and such Terms Agreement shall
inure to the benefit of and be binding upon each Underwriter participating in
the offering of such Securities.

     The Company has filed with the Securities and Exchange Commission (the
"Commission") a registration statement on Form S-3 (No. 33-_____) relating to
the Securities, and the offering thereof from time to time in accordance with
Rule 415 under the Securities Act of 1933, as amended (the "1933 Act"), and has
filed such amendments thereto as may have been required to the date hereof.
Such registration statement has been declared effective by the Commission, and
the Indenture has been qualified under the Trust Indenture Act of 1939, as
amended (the "1939 Act").  Such registration statement and the prospectus
relating to the sale of Securities including all documents incorporated therein
by reference, as from time to time amended or supplemented pursuant to the
Securities Exchange Act of 1934, as amended (the "1934 Act"), the 1933 Act or
otherwise, are referred to herein as the "Registration Statement" and the
"Prospectus", respectively; provided, however, that a supplement to the
Prospectus contemplated by Section 3(a) (a "Prospectus Supplement") shall be
deemed to have supplemented the Prospectus only with respect to the offering of
Securities to which it relates.

                                       2
<PAGE>
 
     SECTION 1.  Representations and Warranties.
                 ------------------------------ 

     (a)  The Company represents and warrants to each of you as of the date
hereof, and to each Underwriter named in a Terms Agreement as of the date
thereof (such latter date, which shall pertain only to the Securities being sold
pursuant to such Terms Agreement, being hereinafter referred to as the
"Representation Date") as follows:

          (i)  The Registration Statement and the Prospectus, at the time the
     Registration Statement became effective and as of the applicable
     Representation Date, complied in all material respects with the
     requirements of the 1933 Act and the rules and regulations thereunder (the
     "1933 Act Regulations") and the 1939 Act.  The Registration Statement, at
     the time it became effective and as of the applicable Representation Date,
     did not, and will not, contain an untrue statement of a material fact or
     omit to state a material fact required to be stated therein or necessary to
     make the statements therein not misleading.  The Prospectus, at the time
     the Registration Statement became effective and as of the applicable
     Representation Date, did not, and will not include an untrue statement of a
     material fact or omit to state a material fact necessary in order to make
     the statements therein, in the light of the circumstances under which they
     were made, not misleading; provided, however, that the representations and
     warranties in this subsection shall not apply to statements in or omissions
     from the Registration Statement or Prospectus made in reliance upon and in
     conformity with information furnished to the Company in writing by any
     Underwriter through you expressly for use in the Registration Statement or
     the Prospectus or to that part of the Registration Statement which shall
     constitute the Statement of Eligibility and Qualification under the 1939
     Act (Form T-1), of the Trustee.

         (ii)  The documents incorporated or deemed to be incorporated by
     reference in the Prospectus, at the time they were or hereafter are filed
     with the Commission, complied and will comply in all material respects with
     the requirements of the 1934 Act and the rules and regulations of the
     Commission under the 1934 Act (the "1934 Act Regulations"), and, when read
     together with the other information in the Prospectus, at the time the
     Registration Statement and any amendments thereto become effective, will
     not contain an untrue statement of a material fact or omit to state a
     material fact required to be stated therein or necessary in order to make
     the statements therein, in the light of the circumstances under which they
     were made, not misleading.

        (iii)  The accountants who certified the financial statements included
     in the Registration Statement are

                                       3
<PAGE>
 
     independent public accountants as required by the 1933 Act and the 1933 Act
     Regulations.

         (iv)  The financial statements of the Company and its consolidated
     subsidiaries incorporated by reference in the Registration Statement and
     the Prospectus present fairly the financial position of the Company and its
     consolidated subsidiaries as at the dates indicated and the results of
     their operations and cash flows for the periods specified; except as
     otherwise stated in the Registration Statement, said financial statements
     have been prepared in conformity with generally accepted accounting
     principles applied on a consistent basis; and the supporting schedules
     included in the Registration Statement present fairly the information
     required to be stated therein.

          (v)  Since the respective dates as of which information is given in
     the Registration Statement and the Prospectus, except as otherwise stated
     therein, (A) there has been no material adverse change in the condition,
     financial or otherwise, or in the earnings, business affairs or business
     prospects of the Company and its subsidiaries considered as one enterprise,
     whether or not arising in the ordinary course of business, (B) there have
     been no transactions entered into by the Company or any of its
     subsidiaries, other than those in the ordinary course of business, which
     are material with respect to the Company and its subsidiaries considered as
     one enterprise, and (C) except for regular quarterly dividends on the
     Company's Common Stock, par value $1.00 per share (the "Common Stock"),
     there has been no dividend or distribution of any kind declared, paid or
     made by the Company on any class of its capital stock.

         (vi)  The Company is a corporation in good standing, duly organized and
     validly existing under the laws of the State of South Dakota with corporate
     power and authority to own, lease and operate its properties and to conduct
     its business as described in the Registration Statement; the Company is
     duly qualified as a foreign corporation for the transaction of business and
     is in good standing under the laws of the States of Wyoming, Montana and
     Nebraska; and the Company's business is such that it is not required to
     qualify as a foreign corporation in any other jurisdiction, except for any
     failures to be so qualified which, taken as a whole, are not material to
     the Company and its subsidiaries considered as one enterprise.

        (vii)  Each subsidiary of the Company is a corporation in good standing,
     duly organized and validly existing under the laws of its jurisdiction of
     incorporation, has due corporate authority to carry on the business in
     which it is engaged and to own and operate the properties used by it in
     such business, as described in the Registration Statement; has

                                       4
<PAGE>
 
     been duly qualified as a foreign corporation for the transaction of
     business and is in good standing under the laws of each other jurisdiction
     in which the failure to so qualify would have a material adverse effect on
     the operations of the Company and its subsidiaries considered as one
     enterprise or result in a material liability to the Company and its
     subsidiaries considered as one enterprise; and all of the issued and
     outstanding capital stock of each such subsidiary has been duly authorized
     and validly issued, is fully paid and non-assessable and is owned by the
     Company, directly or through subsidiaries, free and clear of any mortgage,
     pledge, lien, encumbrance, claim or equitable right.

       (viii)  The Securities have been duly authorized for issuance and sale
     pursuant to this Agreement (or will have been so authorized prior to each
     issuance of Securities) and, when issued, authenticated and delivered
     pursuant to the provisions of this Agreement and of the Indenture against
     payment of the consideration therefor in accordance with this Agreement,
     the Securities will be valid and legally binding obligations of the Company
     enforceable in accordance with their terms, except as enforcement thereof
     may be limited by bankruptcy, insolvency or other laws relating to or
     affecting enforcement of creditors' rights or by general equity principles,
     and will be entitled to the benefits of the Indenture; and the Indenture
     conforms in all material respects to all statements relating thereto
     contained in the Prospectus.

         (ix)  Neither the Company nor any of its subsidiaries is in violation
     of its charter or in default in the performance or observance of any
     material obligation, agreement, covenant or condition contained in any
     material contract, indenture, mortgage, loan agreement, note, lease or
     other instrument to which it is a party or by which it or its property may
     be bound, or to which any of the property or assets of the Company or any
     of its subsidiaries is subject; and the execution, delivery and performance
     of this Agreement, each Terms Agreement and the applicable Supplemental
     Indenture by the Company, and the consummation of the transactions
     contemplated herein and therein have been duly authorized by all necessary
     corporate action and will not conflict with or constitute a breach of, or
     default under, or, other than the lien of the Indenture, result in the
     creation or imposition of any lien, charge or encumbrance upon any property
     or assets of the Company or any of its subsidiaries pursuant to, any
     material contract, indenture, mortgage, loan agreement, note, lease or
     other instrument to which the Company or any of its subsidiaries is a party
     or by which it or any of them may be bound, or to which any of the property
     or assets of the Company or any of its subsidiaries is subject, nor will
     such action result in any violation of the provisions of the charter or
     by-laws of

                                       5
<PAGE>
 
     the Company or any applicable law, administrative regulation or
     administrative or court decree.

         (x)  There is no mortgage, lien, pledge or encumbrance prior to the
     lien of the Indenture on any of the Trust Estate, as defined in the
     Indenture, except the lien of the Indenture and Permitted Encumbrances,
     as defined in Section 4.01 of the Indenture.

        (xi)  No material labor dispute with employees of the Company or any of
     its subsidiaries exists or, to the knowledge of the Company, is imminent.

       (xii)  There is no action, suit or proceeding before or by any court or
     governmental agency or body, domestic or foreign, now pending, or, to the
     knowledge of the Company, threatened, against or affecting the Company or
     any of its subsidiaries, which is required to be disclosed in the
     Registration Statement (other than as disclosed therein), or which might
     result in any material adverse change in the condition, financial or
     otherwise, or in the earnings, business affairs or business prospects of
     the Company and its subsidiaries considered as one enterprise, or which
     might materially and adversely affect the properties or assets thereof or
     which might materially and adversely affect the consummation of this
     Agreement or any Terms Agreement; all pending legal or governmental
     proceedings to which the Company or any subsidiary of the Company is a
     party or of which any of their property is the subject which are not
     described in the Registration Statement, including ordinary routine
     litigation incidental to the business, are, considered in the aggregate,
     not material; and there are no contracts or documents of the Company or any
     of its subsidiaries which are required to be filed as exhibits to the
     Registration Statement by the 1933 Act or by the 1933 Act Regulations which
     have not been so filed.

      (xiii)  The Public Utility Commission of South Dakota (the "South Dakota
     Commission") and the Public Service Commission of Wyoming (the "Wyoming
     Commission") have duly authorized the issuance and sale of the Securities
     on terms consistent with this Agreement, and no other authorization,
     approval or consent of any court or governmental authority or agency is
     necessary in connection with the offering, issuance or sale of the
     Securities hereunder, except such as may be required under the 1933 Act or
     the 1933 Act Regulations, the 1939 Act or state securities laws.

       (xiv)  The Company and its subsidiaries have statutory authority,
     franchises, licenses, rights of way, easements, and consents, free from
     unduly burdensome restrictions and adequate for the conduct of the business
     in which each of them is engaged.

                                       6
<PAGE>
 
         (xv) Except as otherwise stated in the Registration Statement and the
     Prospectus, the Company and each of its subsidiaries has good and valid
     title, free and clear of all liens and encumbrances, to substantially all
     of their permanent fixed properties owned respectively by them.

        (xvi) The Indenture (a) specifically or by general reference describes
     all generating stations, substations, electrical transmission systems and
     electrical distribution systems owned by the Company, and all franchises,
     permits, licenses, power purchase agreements, and material leases to which
     the Company is a party, (b) specifically describes all parcels of land
     owned by the Company except parcels of land which are immaterial to the
     operation of the business of the Company and whose value does not exceed
     $100,000 in the aggregate, and parcels of land disposed of or released from
     the lien of the Indenture in accordance with the terms of the Indenture,
     and (c) conveys to the Trustee all of the real and personal property owned
     by the Company, except property expressly excepted from the lien of the
     Indenture.

       (xvii)  All Federal, state and other tax returns of the Company and its
     subsidiaries required by law to be filed have been duly filed.  All
     Federal, state and local taxes upon the Company and its subsidiaries or
     upon any of their properties or assets which are due and payable have been
     paid.  Federal income tax returns of the Company and its subsidiaries for
     all years through the year ended December 31, 1989, have been audited by
     the Internal Revenue Service, and the results are fully reflected in the
     financial statements included or incorporated by reference in the
     Registration Statement.  The provision on the books of the Company and its
     subsidiaries for Federal income taxes for all fiscal years since 1989 are
     reasonably adequate and the Company does not anticipate substantial
     assessments for such years in addition to the amounts reserved therefor.

      (xviii)  Except as disclosed in the Registration Statement
     (specifically including the section "Environmental Regulation" in the
     Company's Annual Report on Form 10-K for the year ended December 31, 1993),
     the Company is not aware of any material violation by the Company or any of
     its subsidiaries of any Environmental Law, or of any material liability or
     potential material liability on the part of the Company or any of its
     subsidiaries resulting from the presence, use, release, threatened release,
     emission, disposal, pumping, discharge, generation or processing of Waste
     Materials.  "Waste Materials" means (a) any substance, material or waste
     defined, used or listed as a "hazardous waste," "hazardous substance,"
     "toxic substance," or other similar terms as defined or used in any
     Environmental Law, as such Environmental Law may from time to time be
     amended; and (b) any petroleum products, asbestos, lead-based paint,
     polychlorinated biphenyls, flammable explosives or

                                       7
<PAGE>
 
     radioactive materials.  "Environmental Law" means any federal, state or
     local statute, regulation, judgment, order, or authorization relating to
     emissions, discharges, releases or threatened releases of Waste Materials
     into ambient air, surface water, ground water, publicly owned treatment
     works, septic systems or land or otherwise relating to the pollution or
     protection of health or the environment.

        (xix)  The Company has received all governmental regulatory approvals
     and permits required to authorize the Company to construct the 80 MW coal-
     fired electric generating plant referred to in the Registration Statement
     and the Prospectus as Neil Simpson Unit #2.

         (xx)  No person or corporation, which is a "holding company" or a
     "subsidiary of a holding company", within the meaning of such terms as
     defined in the Public Utility Holding Company Act of 1935, directly or
     indirectly owns, controls or holds with power to vote 10% or more of the
     outstanding voting securities of the Company; and the Company is presently
     exempt from the provision of the Public Utility Holding Company Act of 1935
     which would require it to register thereunder.

        (xxi)  This Agreement has been, and, at each Representation Date, the
     applicable Terms Agreement will have been, duly executed and delivered by
     the Company.

     Any certificate signed by any executive officer of the Company and
delivered to you or to counsel for the Underwriters in connection with an
offering of Securities shall be deemed a representation and warranty by the
Company to each Underwriter participating in each offering as to the matters
covered thereby.

     SECTION 2.  Purchase and Sale.  (a)  The several commitments of the
                 -----------------                                      
Underwriters to purchase Offered Securities pursuant to any Terms Agreement
shall be deemed to have been made on the basis of the representations and
warranties herein contained and shall be subject to the terms and conditions
herein set forth.

     (b)  Payment of the purchase price for, and delivery of, any Offered
Securities to be purchased by the Underwriters shall be made at the office of
Brown & Wood, One World Trade Center, New York, New York 10048, or at such other
place as shall be agreed upon by you and the Company, at 10:00 A.M., New York
City time, on the fifth business day (unless postponed in accordance with the
provisions of Section 10) following the date of the applicable Terms Agreement
or at such other time as shall be agreed upon by you and the Company (each such
time and date being referred to as a "Closing Time").  Unless otherwise
specified in the applicable Terms Agreement, payment shall be made to the
Company by certified or official bank check or checks in New York Clearing House
or similar next day funds payable to the order of

                                       8
<PAGE>
 
the Company against delivery to you for the respective accounts of the
Underwriters of the Securities to be purchased by them.  Such Securities shall
be in such denominations and registered in such names as you may request in
writing at least two business days prior to the applicable Closing Time.  Such
Securities, which may be a temporary form, will be made available for
examination and packaging by you on or before the first business day prior to
Closing Time.

     SECTION 3.  Covenants of the Company.  The Company covenants with each of
                 ------------------------                                     
you, and with each Underwriter participating in any applicable offering of
Securities, as follows:

          (a)  Promptly following the execution of each Terms Agreement, the
     Company will prepare a Prospectus Supplement setting forth the principal
     amount of Securities covered thereby, the terms of such Securities not
     otherwise specified in the Prospectus, the names of the Underwriters
     participating in the offering and the principal amount of Securities which
     each severally has agreed to purchase, the names of the Underwriters acting
     as co-managers in connection with the offering, the price at which the
     Securities are to be purchased by the Underwriters from the Company, the
     initial public offering price, the selling concession and reallowances, if
     any, and such other information as you and the Company deem appropriate in
     connection with the offering of the Securities.  The Company will promptly
     transmit copies of the Prospectus Supplement to the Commission for filing
     pursuant to Rule 424 of the 1933 Act Regulations and will furnish to the
     Underwriters named therein as many copies of the Prospectus and such
     Prospectus Supplement as you shall reasonably request.

          (b)  The Company will notify each of you immediately, and confirm the
     notice in writing, (i) of the effectiveness of the Registration Statement
     and any amendment thereto (including any post-effective amendment), (ii) of
     the mailing or the delivery or EDGAR transmission to the Commission for
     filing of any supplement to the Prospectus or any document to be filed
     pursuant to the 1934 Act, (iii) of the receipt of any comments from the
     Commission, (iv) of any request by the Commission for any amendment to the
     Registration Statement or any amendment or supplement to the Prospectus or
     for additional information, and (v) of the issuance by the Commission of
     any stop order suspending the effectiveness of the Registration Statement
     or the initiation of any proceedings for that purpose.  The Company will
     make every reasonable effort to prevent the issuance of any stop order and,
     if any stop order is issued, to obtain the lifting thereof at the earliest
     possible moment.

          (c)  The Company will give you notice of its intention to file or
     prepare any amendment to the Registration Statement (including any post-
     effective amendment) or any

                                       9
<PAGE>
 
     amendment or supplement to the Prospectus, whether pursuant to the 1934
     Act, the 1933 Act or otherwise, and will furnish you with copies of any
     such amendment or supplement or other document proposed to be filed a
     reasonable amount of time prior to such proposed filing and will not file
     any such amendment or supplement or other document or use any such
     prospectus to which you or counsel shall object.

          (d)  The Company will deliver to you as many signed copies of the
     registration statement as originally filed and of each amendment thereto
     (including exhibits filed therewith or incorporated by reference therein)
     as you may reasonably request and will also deliver to each of you a
     conformed copy of the Registration Statement and of each amendment thereto
     for each of the Underwriters.

          (e)  If at any time when the Prospectus is required by the 1933 Act to
     be delivered in connection with sales of the Securities any event shall
     occur or condition exist as a result of which it is necessary, in the
     opinion of your counsel, to further amend or supplement the Prospectus in
     order that the Prospectus will not include an untrue statement of a
     material fact or omit to state any material fact necessary in order to make
     the statements therein not misleading in the light of the circumstances
     existing at the time it is delivered to a purchaser or if it shall be
     necessary, in the opinion of either such counsel, at any such time to amend
     or supplement the Registration Statement or the Prospectus in order to
     comply with the requirements of the 1933 Act or the 1933 Act Regulations,
     the Company will forthwith amend or supplement the Prospectus or make
     appropriate filings under the 1934 Act (in form and substance satisfactory
     to your counsel) so that, as so amended or supplemented, the Prospectus
     will not include an untrue statement of a material fact or omit to state a
     material fact necessary in order to make the statements therein in the
     light of the circumstances existing at the time it is delivered to a
     purchaser, not misleading, and the Company will furnish to you a reasonable
     number of copies of such amendment or supplement or 1934 Act filing.

          (f)  The Company will endeavor, in cooperation with you, to qualify
     the Securities for offering and sale under the applicable securities laws
     of such states and other jurisdictions of the United States as you may
     designate; provided, however, that the Company shall not be obligated to
     qualify as a foreign corporation in any jurisdiction in which it is not so
     qualified.  In each jurisdiction in which the Securities, have been so
     qualified, the Company will file such statements and reports as may be
     required by the laws of such jurisdiction to continue such qualification in
     effect for as long as may be required for the distribution of the
     Securities.  The Company will promptly advise you of the receipt by the
     Company of any notification with respect

                                       10
<PAGE>
 
     to the suspension of the qualification of the Securities for sale in any
     such state or jurisdiction or the initiating or threatening of any
     proceeding for such purpose.

          (g)  The Company will make generally available to its security holders
     (as defined in Rule 158 of the 1933 Act Regulations) as soon as
     practicable, but not later than 90 days after the close of the period
     covered thereby, an earning statement (which need not be audited, but which
     shall be in form complying with the provisions of Rule 158) covering a
     twelve month period beginning not later than the first day of the Company's
     fiscal quarter next following the "effective date" (as defined in said Rule
     158) of the Registration Statement.

          (h)  The Company during the period when the Prospectus is required to
     be delivered under the 1933 Act, will file promptly all documents required
     to be filed with the Commission pursuant to Sections 13(a), 13(c), 14 or
     15(d) of the 1934 Act within the time periods required by the 1934 Act
     Regulations.

          (i)  For such period of time as is specified in a Terms Agreement,
     commencing on the date of such Terms Agreement with respect to the
     Securities covered thereby, the Company will not, without the prior written
     consent of such of you as may be named in such Terms Agreement, directly or
     indirectly, sell, offer to sell, grant any option for the sale of, or
     otherwise dispose of, any Securities.

     SECTION 4.  Payment of Expenses.  The Company will pay all expenses
                 -------------------                                    
incident to the performance of its obligations under this Agreement, including
(i) the printing and filing of the Registration Statement as originally filed
and of each amendment thereto, (ii) the reproduction and delivery of this
Agreement and each Terms Agreement, (iii) the preparation, issuance and delivery
of the certificates for the Securities to the Underwriters, (iv) the fees and
disbursements of the Company's counsel and accountants, (v) the qualification of
the Securities and, if applicable, the Common Stock, under securities laws in
accordance with the provisions of Section 3(f) hereof, including filing fees and
the reasonable fees and disbursements of counsel for the Underwriters in
connection therewith and in connection with the preparation of the Blue Sky
Survey and any Legal Investment Survey (copies of which shall be furnished to
the Company promptly after preparation by such counsel), (vi) the printing and
delivery to the Underwriters of copies of the registration statement as
originally filed and all amendments thereto, the Registration Statement and all
amendments thereto, of each preliminary prospectus, and of the Prospectus and
any amendments or supplements thereto, (vii) the reproduction and delivery to
the Underwriters of copies of each Supplemental Indenture and the Blue Sky
Survey and any Legal Investment Survey, and (viii) the fees of rating agencies.

                                       11
<PAGE>
 
     If a Terms Agreement is terminated by such of you as are named therein in
accordance with the provisions of Section 5 or Section 9(i) hereof, the Company
shall reimburse the Underwriters named in such Terms Agreement for all of their
out-of-pocket expenses, including the reasonable fees and disbursements of
counsel for such Underwriters.

     SECTION 5.  Conditions of Underwriters' Obligations.  The obligations of
                 ---------------------------------------                     
the Underwriters to purchase Securities pursuant to any Terms Agreement are
subject to the accuracy of the representations and warranties of the Company
herein contained, to the accuracy of the statements of the Company's officers
made in any certificate furnished pursuant to the provisions hereof, to the
performance by the Company of all of its covenants and other obligations
hereunder, and to the following further conditions:

          (a)  At the applicable Closing Time (i) no stop order suspending the
     effectiveness of the Registration Statement shall have been issued under
     the 1933 Act or proceedings therefor initiated or threatened by the
     Commission, (ii) the rating assigned by any nationally recognized
     statistical rating organization to any debt securities or preferred stock
     of the Company as of the date of the applicable Terms Agreement shall not
     have been lowered since the execution of such Terms Agreement nor shall any
     such rating organization have publicly announced that it has placed any
     debt securities or preferred stock of the Company on what is commonly
     termed a "watch list" for possible downgrading or (iii) there shall not
     have come to the attention of such of you as may be named in the applicable
     Terms Agreement any facts that would cause such of you to believe that the
     Prospectus, together with the applicable Prospectus Supplement, at the time
     it was required to be delivered to a purchaser of the Securities, contained
     an untrue statement of a material fact or omitted to state a material fact
     necessary in order to make the statements therein, in the light of the
     circumstances existing at such time, not misleading.

          (b)  At the applicable Closing Time you shall have received:

               (1)  The favorable opinion, dated as of the applicable Closing
          Time, of Morrill Brown & Thomas, counsel for the Company, in form and
          substance satisfactory to such of you as may be named in the
          applicable Terms Agreement to the effect that:


                    (i)  The Company is a corporation in good standing, duly
               organized and validly existing under the laws of the State of
               South Dakota and has the corporate power and authority under its

                                       12
<PAGE>
 
               articles of incorporation to own its properties and conduct its
               business as described in the Registration Statement and
               Prospectus.

                  (ii)  The Company and each of its subsidiaries has been duly
               qualified as a foreign corporation for the transaction of
               business and is in good standing under the laws of each
               jurisdiction in which the failure to so qualify by the Company or
               any subsidiary would have a material adverse effect on the
               operations of the Company and its subsidiaries considered as one
               enterprise.

                 (iii)  The Indenture has been duly and validly authorized,
               executed and delivered by the Company and constitutes the valid
               and binding agreement of the Company, enforceable in accordance
               with its terms, except as enforcement thereof may be limited by
               bankruptcy, insolvency or other laws relating to or affecting
               enforcement of creditors' rights or by general equity principles.

                  (iv)  The Securities covered by the applicable Terms
               Agreement are in the form contemplated by the Indenture,
               including the applicable Supplemental Indenture, and have been
               duly and validly authorized by all necessary corporate action
               and, when executed and authenticated as specified in the
               Indenture and delivered against payment pursuant to this
               Agreement, as supplemented by the applicable Terms Agreement,
               will be valid and binding obligations of the Company enforceable
               in accordance with their terms, except as enforcement thereof may
               be limited by bankruptcy, insolvency or other laws relating to or
               affecting enforcement or creditors' rights or by general equity
               principles, and will be entitled to the benefits of the
               Indenture.

                   (v)  The Indenture and the Securities covered by the
               applicable Terms Agreement conform in all material respects to
               the descriptions thereof in the Prospectus and the applicable
               Prospectus Supplement.

                  (vi)  The Indenture is qualified under the 1939 Act.

                 (vii)  This Agreement and the applicable Terms Agreement have
               been duly authorized, executed and delivered by the Company.

                                       13
<PAGE>
 
                (viii)  The Registration Statement is effective under the 1933
               Act and, to the best of their knowledge and information, no stop
               order suspending the effectiveness of the Registration Statement
               has been issued under the 1933 Act or proceedings therefor
               initiated or threatened by the Commission.

                  (ix)  At the time the Registration Statement became effective
               and as of the date of the applicable Terms Agreement, the
               Registration Statement (other than the financial statements and
               schedules and other financial or statistical data included or
               incorporated by reference therein and the Statement of
               Eligibility of the Trustee on Form T-1 (the "Form T-1"), as to
               which no opinion need be rendered) complied as to form in all
               material respects with the requirements of the 1933 Act, the 1939
               Act and the 1933 Act Regulations.

                   (x)  The documents incorporated by reference into the
               Prospectus pursuant to Item 12 of Form S-3 under the 1933 Act
               (other than the financial statements included therein, as to
               which no opinion need be rendered) at the time they were filed
               with the Commission, complied as to form in all material respects
               with the requirements of the 1934 Act and the 1934 Act
               Regulations.

                  (xi)  The South Dakota and Wyoming Commissions have
               authorized the issue and sale of the Securities; such
               authorizations, to the best of such counsel's knowledge, are
               still in force and effect and are sufficient for the issue and
               sale of the Securities; the issue and sale of the Securities are
               in conformity with the terms of such authorizations; and no other
               consent, approval, authorization, order, registration or
               qualification of or with any court or governmental agency or body
               is required for the issue and sale of the Securities by the
               Company or the consummation by the Company of the transactions
               contemplated by this Agreement, except such as may be required
               under the 1933 Act, the 1939 Act or the 1933 Act Regulations or
               state securities laws.

                 (xii)  Each subsidiary of the Company is a corporation in
               good standing, duly organized and validly existing under the laws
               of its jurisdiction of incorporation and has corporate power and
               authority to carry on the business in which it is engaged and to
               own and operate the properties used by it in such business and
               all of

                                       14
<PAGE>
 
               the issued and outstanding capital stock of each subsidiary of
               the Company has been duly authorized and validly issued, is fully
               paid and non-assessable, and is owned directly or indirectly by
               the Company free and clear of liens, encumbrances, equities or
               claims.

                (xiii)  To the best of such counsel's knowledge and
               information, there are no legal or governmental proceedings
               pending or threatened of a character which are required to be
               disclosed in the Registration Statement and Prospectus except as
               otherwise stated therein.

                 (xiv)  The information in the Prospectus under the captions
               "Description of the Offered Bonds" and, with respect to the
               Prospectus Supplement relating to an issue of Securities, any
               further description with respect to such Securities to the extent
               that such information constitutes matters of law, summaries of
               legal matters, documents or proceedings, or legal conclusions,
               has been reviewed by him and is correct in all material respects.

                  (xv)  To the best of such counsel's knowledge, the
               statements in the Company's Annual Report on Form 10-K for the
               year ended December 31, 1993 under "Rate Regulation" and
               "Environmental Regulation," as modified or superseded by any
               subsequently filed documents, have been prepared or reviewed by
               them and are materially correct.

                 (xvi)  To the best of such counsel's knowledge and
               information, there are no contracts, indentures, mortgages, loan
               agreements, notes, leases or other instruments of a character
               required to be described in the Registration Statement or to be
               filed as exhibits thereto other than those described or referred
               to therein or filed or incorporated by reference as exhibits
               thereto, the descriptions thereof or references thereto are
               correct, and no default exists in the due performance or
               observance of any material obligation, agreement, covenant or
               condition contained in any contract, indenture, mortgage, loan
               agreement, note, lease or other instrument so described, referred
               to, or filed or incorporated by reference.

                (xvii)  The execution and delivery of this Agreement and the
               applicable Terms Agreement and Indenture by the Company and the
               consummation by

                                       15
<PAGE>
 
               the Company of the transactions contemplated herein and therein
               will not conflict with or result in a breach or violation of any
               of the terms or provisions of, or constitute a default under,
               indenture, mortgage, deed of trust, loan agreement or other
               agreement or instrument known to such counsel to which the
               Company or any of its subsidiaries is a party or by which the
               Company or any of its subsidiaries may be bound, or to which any
               of the property or assets of the Company or any of its
               subsidiaries is subject, nor will such action result in any
               violation of the provisions of the Articles of Incorporation or
               By-Laws of the Company or any statute or any order, rule or
               regulation known to such counsel of any court or governmental
               agency or body having jurisdiction over the Company or any of its
               subsidiaries or any of their properties; the offering of Consent
               Bonds as contemplated by the Registration Statement and
               Prospectus will not conflict with or constitute a violation of
               the laws of South Dakota.

                (xviii) The Company and its subsidiaries have statutory
               authority, franchises, licenses, rights of way, easements, and
               consents free from unduly burdensome restrictions and adequate
               for the conduct of the business in which each of them is engaged.

                  (xix) Except as set forth in the Registration Statement and
               Prospectus, the Company and its subsidiaries have good and valid
               title to substantially all of their permanent fixed properties
               owned respectively by them.

               (2)  The favorable opinion, dated as of Closing Time, of Brown &
          Wood, counsel for the Underwriters, with respect to the matters set
          forth in (iv), (vi), (vii), (viii) and (ix) of subsection b(1) of this
          Section.

               (3)  In giving their opinions required by subsections (b)(1) and
          (b)(2), respectively, of this Section, Morrill Brown & Thomas and
          Brown & Wood shall each additionally state that nothing has come to
          their attention that would lead them to believe that the Registration
          Statement at the time it became effective, or if an amendment to the
          Registration Statement or an Annual Report on Form 10-K has been filed
          by the Company with the Commission subsequent to the effectiveness of
          the Registration Statement, then at the time of the most recent such
          filing, contained an untrue statement of a material fact or omitted to
          state a material fact required to be stated therein or

                                       16
<PAGE>
 
          necessary to make the statements therein not misleading or that the
          Prospectus, as amended or supplemented at the date of the applicable
          Terms Agreement and at Closing Time, included or includes an untrue
          statement of a material fact or omitted to state a material fact
          necessary in order to make the statements therein, in the light of the
          circumstances under which they were made, not misleading.  In
          connection with the statements required by this subsection (3),
          Morrill Brown & Thomas and Brown & Wood may state that, while they
          have participated in the preparation of the Registration Statement and
          the Prospectus, they have relied upon statements and representations
          of representatives of the Company and have not independently verified
          the statements contained in the Registration Statement or the
          Prospectus.

          (c)  At the applicable Closing Time there shall not have been, since
     the date of the applicable Terms Agreement or since the respective dates as
     of which information is given in the Registration Statement, any material
     adverse change in the condition, financial or otherwise, or in the
     earnings, business affairs or business prospects of the Company and its
     subsidiaries considered as one enterprise, whether or not arising in the
     ordinary course of business, and you shall have received a certificate of
     the President or Vice President or of the Company and of the chief
     financial or chief accounting officer of the Company, dated as of such
     Closing Time, to the effect that (i) there has been no such material
     adverse change, (ii) the representations and warranties of the Company
     contained in Section 1 hereof are true and correct with the same force and
     effect as though expressly made at and as of such Closing Time, (iii) the
     Company has compiled with all agreements and satisfied all conditions on
     its part to be compiled with or satisfied at or prior to such Closing Time,
     and (iv) no stop order suspending the effectiveness of the Registration
     Statement has been issued and no proceedings for that purpose have been
     initiated or threatened by the Commission.

          (d)  At the time of the execution of this Agreement and at the
     applicable Closing Time, you shall have received from Arthur Anderson & Co.
     a letter dated such date, in form and substance satisfactory to you, to the
     effect that (i) they are independent certified public accountants with
     respect to the Company and its subsidiaries within the meaning of the 1933
     Act and the applicable published rules and regulations thereunder; (ii) in
     their opinion the consolidated financial statements and supporting
     schedules audited by them and included or incorporated by reference in the
     Registration Statement comply as to form in all material respects with the
     applicable accounting requirements of the 1933 Act and the related
     published rules and regulations with respect to

                                       17
<PAGE>
 
     registration statement Form S-3 and the 1934 Act and the 1934 Act
     Regulations; (iii) based upon limited procedures set forth in detail in
     such letter (including a review of all interim financial information in
     accordance with SAS No. 71), nothing has come to their attention which
     causes them to believe that (A) the unaudited financial statements and
     supporting schedules of the Company and its subsidiaries included or
     incorporated by reference in the Registration Statement and Prospectus do
     not comply as to form in all material respects with the applicable
     accounting requirements of the 1934 Act and the 1934 Act Regulations or are
     not fairly presented in conformity with generally accepted accounting
     principles applied on a basis substantially consistent with that of the
     audited financial statements included in the Registration Statement, (B) at
     a specified date not more than five days prior to the date of such letter,
     there has been any change in the capital stock or long-term debt of the
     Company consolidated (except for such number of shares of Common Stock set
     forth in said letter as may have been issued pursuant to the Company's
     existing stock option plans and employee stock purchase plan) consolidated
     net current assets or as compared with the amounts shown in the most recent
     balance sheet included or incorporated by reference in the Registration
     Statement or, during the period from a specified date not more than five
     days prior to the date of such letter, there were any decreases, as
     compared with the corresponding period in the preceding year, in
     consolidated net revenue, or in the total or per share amounts of
     consolidated net income of the Company and its subsidiaries except in all
     instances for changes, increases or decreases which the Registration
     Statement and the Prospectus disclose have occurred or may occur; and (iv)
     in addition to the examination referred to in their opinions and the
     limited procedures referred to in clause (iii) above, they have carried out
     certain specified procedures, not constituting an audit, with respect to
     certain amounts, percentages and financial information which are included
     in the Registration Statement and Prospectus and which are specified by you
     and have found such amounts, percentages and financial information to be in
     agreement with the relevant accounting, financial and other records of the
     Company and its subsidiaries identified in such letter.

          (e)  At the applicable Closing Time, counsel for the Underwriters
     shall have been furnished with such documents and opinions as they may
     reasonably require for the purpose of enabling them to pass upon the
     issuance and sale of the Securities as herein contemplated and related
     proceedings, or in order to evidence the accuracy and completeness of any
     of the representations or warranties, or the fulfillment of any of the
     conditions, herein contained; and all proceedings taken by the Company in
     connection with the issuance and sale of the Securities as herein
     contemplated shall be

                                       18
<PAGE>
 
     reasonably satisfactory in form and substance to you and counsel for the
     Underwriters.

    If any condition specified in this Section shall not have been fulfilled
when and as required to be fulfilled, the applicable Terms Agreement may be
terminated by such of you as may be named in such Terms Agreement by notice to
the Company at any time at or prior to the applicable Closing Time, and such
termination shall be without liability of any party to any other party except as
provided in Section 4 hereof.

     SECTION 6.  Indemnification.
                 --------------- 

     (a)  The Company agrees to indemnify and hold harmless each Underwriter and
each person, if any, who controls any Underwriter within the meaning of Section
15 of the 1933 Act as follows:

          (i)  against any and all loss, liability, claim, damage and expense
     whatsoever, as incurred, arising out of any untrue statement or alleged
     untrue statement of a material fact contained in the Registration Statement
     (or any amendment thereto) or incorporated documents, or the omission or
     alleged omission therefrom of a material fact required to be stated therein
     or necessary to make the statements therein not misleading or arising out
     of any untrue statement or alleged untrue statement of a material fact
     contained in the Prospectus (or any amendment or supplement thereto) or the
     omission or alleged omission therefrom of a material fact necessary in
     order to make the statements therein, in the light of the circumstances
     under which they were made, not misleading;

         (ii)  against any and all loss, liability, claim, damage and expense
     whatsoever, as incurred, to the extent of the aggregate amount paid in
     settlement of any litigation, or any investigation or proceeding by any
     governmental agency or body, commenced or threatened, or of any claim
     whatsoever based upon any such untrue statement or omission, or any such
     alleged untrue statement or omission, if such settlement is effected with
     the written consent of the Company; and

        (iii)  against any and all expense whatsoever, as incurred (including,
     the fees and disbursements of counsel chosen by you), reasonably incurred
     in investigating, preparing or defending against any litigation, or any
     investigation or proceeding by any governmental agency or body, commenced
     or threatened, or any claim whatsoever based upon any such untrue statement
     or omission, or any such alleged untrue statement or omission, to the
     extent that any such expense is not paid under (i) or (ii) above;

provided, however, that this indemnity agreement shall not apply to any loss,
- --------  -------                                                            
liability, claim, damage or expense to the extent

                                       19
<PAGE>
 
arising out of any untrue statement or omission or alleged untrue statement or
omission made in reliance upon and in conformity with written information
furnished to the Company by any Underwriter through you expressly for use in the
Registration Statement (or any amendment thereto) or the Prospectus (or any
amendment or supplement thereto).

     (b)  Each Underwriter agrees, severally and not jointly, to indemnify and
hold harmless the Company, its directors, each of its officers who signed the
Registration Statement, and each person, if any, who controls the Company within
the meaning of Section 15 of the 1933 Act against any and all loss, liability,
claim, damage and expense described in the indemnity contained in subsection (a)
of this Section, as incurred, but only with respect to untrue statements or
omissions, or alleged untrue statements or omissions, made in the Registration
Statement (or any amendment thereto) or the Prospectus (or any amendment or
supplement thereto) in reliance upon and in conformity with written information
furnished to the Company by such Underwriter through you expressly for use in
the Registration Statement (or any amendment thereto) or the Prospectus (or any
amendment or supplement thereto).

     (c)  Each indemnified party shall give notice as promptly as reasonably
practicable to each indemnifying party of any action commenced against it in
respect of which indemnity may be sought hereunder, but failure to so notify an
indemnifying party shall not relieve such indemnifying party from any liability
which it may have otherwise than on account of this indemnity agreement.  If any
such claim or action shall be brought against an indemnified party, and it shall
notify the indemnifying party thereof, the indemnifying party shall be entitled,
at its own expense, to participate therein and to the extent that it wishes,
jointly with any other similarly notified indemnifying party, to assume the
defense thereof with counsel reasonably satisfactory to the indemnified party.
After notice from the indemnifying party to the indemnified party of its
election to assume the defense of such claim or action, the indemnifying party
shall not be liable to the indemnified party under this Section for any legal or
other expenses subsequently incurred by the indemnified party in connection with
the defense thereof other than reasonable costs of investigation; provided,
however, that you shall have the right to employ counsel to represent jointly
you and the other Underwriters and their respective controlling persons who may
be subject to liability arising out of any claim in respect of which indemnity
may be sought by you and the other Underwriters against the Company under this
Section if, in your reasonable judgment, it is advisable for you and the other
Underwriters and controlling persons to be jointly represented by separate
counsel and in that event the fees and expenses of such separate counsel shall
be paid by the Company.  In no event shall the indemnifying parties be liable
for fees and expenses of more than one counsel (in addition to any local
counsel) separate from their own counsel for all indemnified parties in
connection with

                                       20
<PAGE>
 
any one action or separate but similar or related actions in the same
jurisdiction arising out of the same general allegations or circumstances.

     SECTION 7.  Contribution.  In order to provide for just and equitable
                 ------------                                             
contribution in circumstances in which the indemnity agreement provided for in
Section 6 hereof is for any reason held to be unenforceable by the indemnified
parties although applicable in accordance with its terms, the Company and the
Underwriters of each offering of Securities shall contribute to the aggregate
losses, liabilities, claims, damages and expenses of the nature contemplated by
said indemnity agreement incurred by the Company and one or more of the
Underwriters in respect of such offering, as incurred, in such proportions that
the Underwriters are responsible for that portion represented by the percentage
that the underwriting discount appearing on the cover page of the Prospectus in
respect of such offering bears to the initial public offering price appearing
thereon and the Company is responsible for the balance; provided, however, that
no person guilty of fraudulent misrepresentation (within the meaning of Section
11(f) of the 1933 Act) shall be entitled to contribution from any person who was
not guilty of such fraudulent misrepresentation.  For purposes of this Section,
each person, if any, who controls an Underwriter within the meaning of Section
15 of the 1933 Act shall have the same rights to contribution as such
Underwriter, and each director of the Company, each officer of the Company who
signed the Registration Statement, and each person, if any, who controls the
Company within the meaning of Section 15 of the 1933 Act shall have the same
rights to contribution as the Company.

     SECTION 8.  Representations, Warranties and Agreements to Survive Delivery.
                 -------------------------------------------------------------  
All representations, warranties and agreements contained in this Agreement or
any Terms Agreement or contained in certificates of officers of the Company
submitted pursuant hereto, shall remain operative and in full force and effect,
regardless of any investigation made by or on behalf of any Underwriter or
controlling person, or by or on behalf of the Company, and shall survive
delivery of any Securities to the Underwriters.

     SECTION 9.  Termination of Agreement.  This Agreement may be terminated for
                 ------------------------                                       
any reason at any time by either the Company or you upon the giving of thirty
days' written notice of such termination to the other parties hereto.  Such of
you as may be named in any Terms Agreement may also terminate such Terms
Agreement, immediately upon by notice to the Company, at any time at or prior to
the applicable Closing Time (i) if there shall have been, since the date of such
Terms Agreement or since the respective dates as of which information is given
in the Registration Statement, any material adverse change in the condition,
financial or otherwise, or in the earnings, business affairs or business
prospects of the Company and its subsidiaries considered as one enterprise,
whether or not arising in the

                                       21
<PAGE>
 
ordinary course of business, or (ii) if there shall have occurred any material
adverse change in the financial markets in the United States or any outbreak or
escalation of hostilities or other national or international calamity or crisis,
the effect of which is such as to make it, in the reasonable judgment of such of
you as are named in such Terms Agreement, impracticable to market the Securities
or to enforce contracts for the sale of the Securities, or (iii) if trading in
the Common Stock shall have been suspended by the Commission or a national
securities exchange, or if trading generally on the New York Stock Exchange
shall have been suspended, or minimum or maximum prices for trading shall have
been fixed, or maximum ranges for prices for securities shall have been
required, by said Exchange or by order of the Commission or any other
governmental authority, or if a banking moratorium has been declared by either
Federal New York authorities, or (iv) if the rating assigned by any nationally
recognized statistical rating organization to any debt securities or preferred
stock of the Company as of the time any applicable Terms Agreement was entered
into shall have been lowered since that time or if any such rating organization
shall have publicly announced that it has placed any debt securities or
preferred stock of the Company on what is commonly termed a "watch list" for
possible downgrading.  In the event of any such termination, (x) the covenants
set forth in Section 3 with respect to any offering of Securities shall remain
in effect so long as any Underwriter owns any such Securities purchased from the
Company pursuant to the applicable Terms Agreement and (y) the covenant set
forth in Section 3(h), the provisions of Section 4, the indemnity agreement set
forth in Section 6, the contribution provisions set forth in Section 7 and the
provisions of Sections 8 and 13 shall remain in effect.

     SECTION 10.  Default by One or More of the Underwriters.  If one or more of
                  ------------------------------------------                    
the Underwriters participating in an offering of Securities shall fail at the
applicable Closing Time to purchase the Offered Securities which it or they are
obligated to purchase hereunder and under the applicable Terms Agreement (the
"Defaulted Securities"), then such of you as are named therein shall have the
right, within 24 hours thereafter, to make arrangements for one or more of the
non-defaulting Underwriters, or any other underwriters, to purchase all, but not
less than all, of the Defaulted Securities in such amounts as may be agreed upon
and upon the terms herein set forth; if, however, you shall not have completed
such arrangements within such 24 hour period, then:

          (a)  if the aggregate principal amount of Defaulted Securities does
     not exceed 10% of the aggregate principal amount of Offered Securities to
     be purchased pursuant to such Terms Agreement, the non-defaulting
     Underwriters named in such Terms Agreement shall be obligated to purchase
     the full amount thereof in the proportions that their respective
     underwriting obligations bear to the underwriting obligations of all non-
     defaulting Underwriters, or

                                       22
<PAGE>
 
          (b)  if the aggregate principal amount of Defaulted Securities
     exceeds 10% of the aggregate principal amount of Offered Securities to be
     purchased pursuant to such Terms Agreement, the applicable Terms Agreement
     shall terminate without liability on the part of any non-defaulting
     Underwriter.

     No action taken pursuant to this Section shall relieve any defaulting
Underwriter from liability in respect of its default under this Agreement and
the applicable Terms Agreement.

     In the event of any such default by any Underwriter or Underwriters as set
forth in this Section, either you or the Company shall have the right to
postpone the applicable Closing Time for a period not exceeding seven days in
order to effect any required changes in the Registration Statement or Prospectus
or in any other documents or arrangements.

     SECTION 11.  Notices.  All notices and other communications hereunder shall
                  -------                                                       
be in writing and shall be deemed to have been duly given if mailed or
transmitted by any standard form of telecommunication.  Notices to the
Underwriters shall be directed to you at the address set forth above or, in
respect of any Terms Agreement, to such other person and place as my be
specified therein; notices to the Company shall be directed to it at Black Hills
Corporation, P.O. Box 1400, Rapid City, South Dakota, 57709, attention Dale E.
Clement, Senior Vice President -- Finance.

     SECTION 12.  Parties.  This Agreement shall inure to the benefit of and be
                  -------                                                      
binding upon you and the Company and any Underwriter who becomes a party to a
Terms Agreement, and their respective successors.  Nothing expressed or
mentioned in this Agreement or a Terms Agreement is intended or shall be
construed to give any person, firm or corporation, other than the parties hereto
and thereto and their respective successors and the controlling persons and
officers and directors referred to in Sections 6 and 7 and their heirs and legal
representatives, any legal or equitable right, remedy or claim under or in
respect of this Agreement or a Terms Agreement or any provision herein or
therein contained.  This Agreement and any Terms Agreement and all conditions
and provisions hereof and thereof are intended to be for the sole and exclusive
benefit of the parties and their respective successors and said controlling
persons and officers and directors and their heirs and legal representatives,
and for the benefit of no other person, firm or corporation.  No purchaser of
Securities from any Underwriter shall be deemed to be a successor by reason
merely of such purchase.

    SECTION 13.  Governing Law.  This Agreement and each Terms Agreement 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 in said State.

                                       23
<PAGE>
 
     If the foregoing is in accordance with your understanding of our agreement,
please sign and return to the Company a counterpart hereof, whereupon this
instrument, along with all counterparts, will become a binding agreement between
you and the Company in accordance with its terms.

                                    Very truly yours,

                                    BLACK HILLS CORPORATION



                                    By_____________________________
                                       Name:
                                       Title:
CONFIRMED AND ACCEPTED,
  as of the date first above written:



MERRILL LYNCH & CO.
Merrill Lynch, Pierce, Fenner & Smith
            Incorporated
PAINEWEBBER INCORPORATED
By  MERRILL LYNCH & CO.
    Merrill Lynch, Pierce, Fenner & Smith
                Incorporated


By_____________________________
     Authorized Signatory

                                       24
<PAGE>
 
                                                                       Exhibit A



                            BLACK HILLS CORPORATION

                              First Mortgage Bonds

                                TERMS AGREEMENT
                                ---------------

                                                    Dated:            , 19

To:
 
 

Re:  Underwriting Agreement dated               , 19  .

Title of Securities:
Current ratings:
Interest rate:       %.   Payable:
Date of Maturity:
Public offering price:              %, plus accrued interest or amortized
                                    original issue discount, if any, from
                                    __________, 19__.
Purchase price:                     %, plus accrued interest or amortized
                                    original issue discount, if any, from
                                    _________, 19___ (payable in next day
                                    funds).

Closing date and location:
Additional co-managers, if any:
Redemption provisions:
Sinking fund requirements:
Number of days for which the
  provisions of Section 3(i)
  apply:

Each Underwriter severally agrees, subject to the terms and provisions of the
above referenced Underwriting Agreement, which is incorporated herein in its
entirety and made a part hereof, to purchase the principal amount of Offered
Securities set forth opposite its name.

                                       1
<PAGE>
 
                                            Principal Amount of
Name                                        Offered Securities
- ----                                        -------------------


 
                                                 ---------
                    Total...........            $
                                                 =========

                    [Managing Underwriters]


                    By___________________________

                    Acting on behalf of themselves and
                    the other named Underwriters



Accepted:

BLACK HILLS CORPORATION


By________________________
  Name:
  Title:

                                       2

<PAGE>
 
                                                              EXHIBIT 4.04


No. ______________________                         $________________


                            BLACK HILLS CORPORATION

         FIRST MORTGAGE BOND, SERIES ___, _____%, DUE __________, _____

     BLACK HILLS CORPORATION (hereinafter called the "Company"), a corporation
organized and existing under the laws of the State of South Dakota, for value
received, hereby promises to pay to                                         ,
or registered assigns, on the ____ day of ________, _____, at the principal
corporate trust office of the Trustee, in the Borough of Manhattan, The City of
New York,                              Dollars, in any coin or currency of the
United States of America which at the time of payment shall be legal tender for
the payment of public and private debts, and to pay interest thereon from the
date hereof, at the rate of _____ percent, per annum (computed on the basis of a
360-day year of 12 thirty-day months), payable at said principal office of the
Trustee in like coin or currency semi-annually on ___________ and ______________
in each year until the principal hereof shall have become due and payable, and
thereafter if default be made in the payment of such principal, at the rate of
_____ percent, per annum until the principal hereof shall be paid.  The first
interest payment shall be due ___________, ____ and shall include interest from
the date of initial authentication and delivery thereof.

     This Bond is one of an authorized issue of Bonds of the Company known as
its "First Mortgage Bonds", issued and to be issued in one or more series under,
and all equally and ratably secured (except as any sinking, amortization,
improvement, renewal or other analogous fund, established in accordance with the
provisions of the Indenture hereinafter mentioned, may afford additional
security for the Bonds of any particular series) by an Indenture of Mortgage and
Deed of Trust dated as of September 1, 1941 executed by the Company to Central
Hanover Bank and Trust Company (subsequently known as The Hanover Bank and
herein, with its successor by merger, Chemical Bank, called the "Trustee"), as
Trustee, as supplemented and amended by Supplemental Indentures dated as of July
15, 1945, January 15, 1948, January 15, 1949, March 1, 1950, March 1, 1952, July
1, 1956, May 1, 1957, May 1, 1959, April 1, 1960, August 1, 1960, June 1, 1961,
October 1, 1962, May 1, 1963, June 1, 1969, June 15, 1974, August 1, 1974, July
15, 1975, May 1, 1976, February 15, 1977, April 1, 1977, June 1, 1977, July 14,
1982, September 1, 1986, April 13, 1987, June 15, 1988, May 15, 1991, June 1,
1991 and ____________________ (said Original Indenture as so supplemented and
amended being hereinafter collectively called the "Indenture"), to which
Indenture and all further instruments supplemental thereto reference is hereby
made for a description of the properties mortgaged and pledged, the nature and
extent of the security, the rights of the holders of said Bonds and the coupons
appurtenant to coupon Bonds, if any, and of the Trustee and of the Company in
respect of such security, and the terms and conditions upon which said Bonds are
and are to be issued and secured.

     As provided in the Indenture, said Bonds are issuable in series which may
vary as in the Indenture provided or permitted.  This Bond is one of a series of
Bonds authorized by

<PAGE>
 
the ___________________ Supplemental Indenture and entitled "First Mortgage
Bonds, Series ___, ______%, due __________, ____" (the "Series ____ Bonds").

     To the extent permitted by the Indenture and as provided therein, with the
consent of the Company and upon the written consent or affirmative vote of at
least sixty-six and two-thirds percent in principal amount of the Bonds then
outstanding and entitled to consent, and of not less than sixty-six and two-
thirds percent, in principal amount of the Bonds then outstanding and entitled
to consent of each series affected thereby in case one or more but less than all
of the series of Bonds issued under the Indenture are so affected, the rights
and obligations of the Company and of the holders of Bonds and coupons
appurtenant to coupon Bonds, and the terms and provisions of the Indenture and
of any instrument supplemental thereto may be modified from time to time,
provided that no such modification or alteration shall be made which would
postpone the date fixed herein or in the Indenture for the payment of the
principal of, or any installment of interest on, the Bonds, or reduce the
principal of, or the rate of interest payable on, the Bonds, or reduce the
percentage of the principal amount of Bonds the consent of which is required for
the authorization of any such modification or alteration, without the consent of
all of the holders affected thereby.  The rights, duties or immunities of the
Trustee shall not be modified without the written consent of the Trustee.

     Consent Bonds.
     ------------- 

     The Holders, including any successor Holders, of these Series _____ Bonds
to be issued under the terms of the _______________ Supplemental Indenture, by
becoming such Holders shall be deemed to have consented to the Twenty Eighth
Supplemental Indenture ("Proposed Supplement"), a copy of which is attached as
Exhibit A to the _____________ Supplemental Indenture authorizing this Series of
Bonds.  This provision does hereby constitute a written consent of the Holders,
including all successor Holders, of the Series ____ Bonds to the execution and
adoption of the Proposed Supplement under the provisions of Section 18.11 of the
Indenture, and such consent is received by the Trustee as a consent for the
Trustee to execute the Proposed Supplement in lieu of the holding of a meeting
of Bondholders pursuant to Article Eighteen of the Indenture.

     Optional Redemption.
     ------------------- 

     [To be included if the Series includes optional redemption provisions]

     The Bonds of this Series are subject to redemption, in whole or in part, at
the option of the Company on (but not before) ______________, _____ or at any
time thereafter, upon at least thirty (30) days and not more than fifty (50)
days notice, mailed to all registered owners of the Bonds of this Series to be
redeemed, at their respective addresses as the same shall appear on the Bond
register of the Company, all subject to the conditions and as more fully
provided in the Indenture, upon payment of accrued interest to the redemption
date plus the applicable percentage of the principal amount thereof set forth
below:

                                       2
<PAGE>
 
                     If Redeemed During
                       12-Month Period
                          Beginning                Percentage
                      ----------------             ----------

 



     Mandatory sinking fund.
     -----------------------

   [To be included if the Series includes mandatory sinking fund provisions]

     The Bonds of Series ____ are subject to redemption, pursuant to the terms
of the mandatory sinking fund provided for in the ____________ Supplemental
Indenture, on ____________, ______ and on each ____________ thereafter, at 100%
of the principal amount thereof plus accrued interest to the redemption date in
the following respective principal amounts:

              Date                      Principal Amount
              ----                      ----------------

 


No premium shall be payable in connection with any payment made pursuant to this
Section.

     Pursuant to the provisions of Section 8.08 of the Indenture, the Series
____ Bonds are further subject to redemption, in whole or in part, by
application of monies deposited with the Trustee in certain cases for the
release of properties from the lien of the Indenture, at any time with the
giving of the requested notice.  Monies applied to the redemption of Series ____
Bonds pursuant to Section 8.08 of the Indenture on or before ______________,
_____ shall be applied at a redemption price equal to 100% of the principal
amount of the Bonds of Series ___ to be redeemed, plus accrued interest to the
redemption date.  Monies applied to the redemption of Series ___ Bonds pursuant
to the provisions of Section 8.08 of the Indenture on or after ___________,
_____, shall be applied at a redemption price equal to the applicable percentage
of the principal amount of the Series ___ Bonds to be redeemed set forth under
the Option Redemption paragraph of this Series ___ Bond plus accrued interest to
the redemption date.

      [Redemption provisions to be consistent with any Optional Redemption
                           provisions of the Series.]

     The particular Series ____ Bonds to be redeemed from time to time shall be
selected by the Trustee in the amount of $1,000 or an integral multiple thereof
and as nearly as practicable pro rata among the registered holders of such
Series ___ Bonds according to the respective principal amounts of such Bonds.

                                       3
<PAGE>
 
     If this Bond or any portion thereof ($1,000 or a multiple) shall be duly
called for redemption as provided in the Indenture, this Bond or such portion
thereof shall (unless the Company shall default in the payment of the redemption
price) cease to bear interest from and after the date fixed for redemption.

     Upon any partial redemption of this Bond, this Bond may, at the option of
the registered holder hereof, be either (a) surrendered to the Trustee in
exchange for one or more new Bonds of this Series for the principal amount of
the unredeemed portion of this Bond or (b) submitted to the Trustee for notation
hereon by the Trustee of the payment of the portion of the principal hereof so
called for redemption.

     If an Event of Default, as defined in the Indenture, shall occur, the
principal of this Bond may become or be declared due and payable, in the manner
and with the effect provided in the Indenture.

     This Bond is transferable by the registered owner hereof in person or by
attorney authorized in writing, at said principal corporate trust office of the
Trustee, upon surrender for cancellation of this Bond and on payment of charges,
and upon any such transfer a new Bond or Bonds, of the same series, for the same
aggregate principal amount, will be issued to the transferee in exchange
herefor.

     First Mortgage Bonds, Series ___, ______%, due _________, _____, are
issuable as fully registered Bonds without coupons of the denominations of
$1,000 and any multiple of $1,000 which may be executed by the Company and
delivered to the Trustee for authentication and delivery.  The Series ___ Bonds,
upon surrender thereof to the Trustee at its principal corporate trust office in
the Borough of Manhattan, The City of New York, are exchangeable for other Bonds
of the same series in such authorized denomination or denominations in the same
aggregate principal amount, as may be requested by the holders surrendering the
same.

     The Company and the Trustee may deem and treat the person in whose name
this Bond is registered as the absolute owner hereof, for the purpose of
receiving payment of or on account of the principal hereof and interest due
hereon, and neither the Company nor the Trustee shall be affected by any notice
to the contrary.

     No recourse shall be had for the payment of the principal of or the
interest on this Bond, or for any claim based hereon or otherwise in respect
hereof or of the Indenture or of any indenture supplemental thereto, against any
incorporator, stockholder, director or officer, as such, past, present or
future, of the Company or of any predecessor or successor corporation, either
directly or through the Company or any predecessor or successor corporation,
whether by virtue of any constitution, statute or rule of law, or by the
enforcement of any assessment or penalty or by any legal or equitable proceeding
or otherwise howsoever; all such liability being, by the acceptance hereof and
as a part of the consideration for the issuance hereof, expressly waived and
released by every holder hereof, as more fully provided in the Indenture;
provided, however, that nothing herein or in the Indenture contained shall be
taken to prevent recourse to and the enforcement of the

                                       4
<PAGE>
 
liability, if any, of any shareholder or any stockholder or subscriber to
capital stock upon or in respect of shares of capital stock not fully paid up.

     This Bond shall not be valid or become obligatory for any purpose until the
certificate of authentication hereon shall have been signed by the Trustee, or
its successor as Trustee, under the Indenture.

     IN WITNESS WHEREOF, the Company has caused this Bond to be signed in its
name by its President or one of its Vice Presidents, and its corporate seal to
be impressed or imprinted hereon and attested by its Secretary or one of its
Assistant Secretaries.

     Dated:                     BLACK HILLS CORPORATION,


                                By___________________________________________

                                  President

ATTEST:

- ------------------------------------
Secretary

               (FORM OF TRUSTEE'S CERTIFICATE OF AUTHENTICATION)

     This is one of the Bonds, of the series designated therein, described in
the within mentioned Indenture.

                                CHEMICAL BANK, as Trustee,


                                By___________________________________________

                                  Authorized Officer

                                       5

<PAGE>
 
- -----------------------------------------------------------------------------
- -----------------------------------------------------------------------------
 
                     BLACK HILLS POWER AND LIGHT COMPANY

                                      TO

                    CENTRAL HANOVER BANK AND TRUST COMPANY

                                                                  As Trustee
                                
                                --------------
 
                            Supplemental Indenture

                          Dated as of March 1, 1950

                                --------------
 
                  Supplemental to Indenture of Mortgage and
                 Deed of Trust Dated as of September 1, 1941

- ------------------------------------------------------------------------------
- ------------------------------------------------------------------------------
                 
<PAGE>
 
    SUPPLEMENTAL INDENTURE, dated as of the 1st day of March, 1950 between BLACK
HILLS POWER AND LIGHT COMPANY, a corporation organized and existing under
the laws of the State of South Dakota (hereinafter called the "Company"),
party of the first part, and CENTRAL HANOVER BANK AND TRUST COMPANY, a
corporation organized and existing under the laws of the State of New York,
as Trustee under the Indenture hereinafter mentioned (hereinafter called the
"Trustee"), party of the second part. 


    WHEREAS, in order to secure an authorized issue of First Mortgage Bonds of 
the Company, the Company has executed and delivered an Indenture of Mortgage
and Deed of Trust to the Trustee, dated September 1, 1941, hereinafter 
referred to as the "Original Indenture", and has also executed and delivered
to said Trustee an indenture dated July 15, 1945, hereinafter referred to as
the "First Supplemental Indenture", an indenture dated January 15, 1948,
hereinafter referred to as the "Second Supplemental Indenture", and an
indenture dated January 15, 1949, hereinafter referred to as the "Third
Supplemental Indenture", supplementing and amending said Original Indenture
which, as so supplemented and amended, is hereinafter referred to as the
"Indenture"; and

    WHEREAS, the Original Indenture was recorded in the Office of the Secretary
of State, South Dakota, October 29, 1941, in Volume 15, Pages 1 to 53, 
inclusive, of the Record of Railway Deeds, Mortgages and Leases and in the 
Office of the Register of Deeds, Weston County, Wyoming, on March 26, 1946, in 
Book 30 of Mortgages, Page 322, and in the office of the County Clerk and 
Ex-Officio Register of Deeds, Crook County, Wyoming, on March 12, 1948, in 
Book 105 at Page 333; the First Supplemental Indenture was recorded in the
Office of the Secretary of State, South Dakota, on July 25, 1945, in Volume
16 at Page 260 of the Record of Railway Deeds, Mortgages and Leases and in
the Office of the Register of Deeds, Weston County, Wyoming, on April 4,
1946, in Book 30 of Mortgages at Page 454 and in the Office of the County
Clerk and Ex-Officio Register of Deeds, Crook County, Wyoming, on March 12,
1948, in Book 105 at Page 333; the Second Supplemental Indenture was recorded
in the Office of the Secretary of State, South Dakota, on March 8, 1948, in
Volume 19 at Page 217, of the Record of Railway Deeds, Mortgages and Leases,
and in the 
<PAGE>
 
                              2
 
Office of the Register of Deeds, Weston County, Wyoming, on March 8, 1948, in 
Book 31 of Mortgages, at Page 603, and in the Office of the County Clerk and 
Ex-Officio Register of Deeds, Crook County, Wyoming, on March 12, 1948, in 
Book 105, at Page 333; and the Third Supplemental Indenture was recorded in 
the Office of the Secretary of State, South Dakota on April 13, 1949 in Volume
20 at Page 63 of the Record of Railway Deeds, Mortgages and Leases and in the 
Office of the Register of Deeds, Weston County, Wyoming, on April 14, 1949 in 
Book 33 of Mortgages at Page 196 and in the Office of the County Clerk and 
Ex-Officio Register of Deeds, Crook County, Wyoming on April 13, 1949 in Book 
105 at Page 545; and

    WHEREAS, pursuant to the provisions of the Indenture, First Mortgage 
Bonds, Series A, 3 3/8%, due September 1, 1971, First Mortgage Bonds, Series 
B, 3%, due July 15, 1975, First Mortgage Bonds, Series C, 3 3/8%, due July 15, 
1975 and First Mortgage Bonds Series D, 3 3/8%, due January 15, 1979 have 
been duly issued and are presently outstanding and secured by the Indenture; 
and 


    WHEREAS, the Company has purchased, constructed or otherwise acquired,
subsequent to the date of execution and delivery of the Third Supplemental
Indenture, certain Property Additions not heretofore specifically mortgaged
and pledged under the Indenture and it desires, by the inclusion of the 
descriptions thereof in this Supplemental Indenture, to specifically mortgage
and pledge such property; and 


    WHEREAS, as permitted by the Indenture the Company by resolutions of its
Board of Directors duly adopted, has determined to create a new series of
bonds to be known as its "First Mortgage Bonds, Series E, 3%" (hereinafter
called "Bonds of Series E"), to be initially authenticated and delivered in
the aggregate principal amount of $990,000, in the form, having the
characteristics and being entitled to the benefits as in this Supplemental
Indenture provided, the Bonds of Series A, Series B, Series C, Series D and
Series E being hereinafter referred to collectively as the "Bonds"; and

    WHEREAS, the Company deems it advisable that the Indenture be amended as
herein provided and the holder of all of the outstanding bonds issued under the 
Indenture, being all of the Bonds of Series A, 
<PAGE>
 
                                      3
 
Series B, Series C and Series D has duly consented, pursuant to the provisions
of Section 18.11 of the Indenture, to the execution of this Supplemental
Indenture and has lodged with the Trustee an instrument duly setting forth
such consent; and

    WHEREAS, the Company, in the exercise of the powers and authority conferred
upon and reserved to it under and by virtue of the provisions of the Original
Indenture, and particularly the provisions contained in Articles Two, 
Seventeen and Eighteen thereof, and pursuant to appropriate resolutions of its
Board of Directors, has duly resolved and determined to make, execute and
deliver to the Trustee a Supplemental Indenture in the form hereof for the
purposes herein provided; and

    WHEREAS, all conditions and requirements necessary to make this Supplemental
Indenture a valid, binding and legal instrument in accordance with its terms
have been done, performed and fulfilled, and the execution and delivery hereof
have been in all respects duly authorized:

    NOW, THEREFORE, THIS INDENTURE WITNESSETH: That the Company, in 
consideration of the premises and of one dollar to it duly paid by the Trustee
at or before the ensealing and delivery of these presents, the receipt whereof
is hereby acknowledged, and of other good and valuable consideration, in order
to better secure the payment both of the principal of and interest on all
Bonds issued under the Indenture and that may be issued under this or any
other indentures supplemental thereto, according to their tenor and effect,
and the performance by the Company of all the covenants and conditions herein
and therein contained, and in order to establish the terms of the Bonds of
Series E, and to amend certain provisions of the Indenture, hereby further
covenants and agrees to and with the Trustee and its successors in the trust
under the Indenture for the benefit of all those who shall from time to time
hold the Bonds and interest coupons, if any, pertaining thereto as follows:

    The Company does hereby ratify and confirm its Mortgage and Pledge to 
the Trustee of all property described in the Indenture and does hereby by these 
presents grant, bargain, sell, warrant, alien, 
<PAGE>
 
                                      4
 
remise, release, convey, confirm, assign, mortgage, pledge and set over unto
the Trustee, and to its successors and assigns forever, the following
described property, constituting property acquired by the Company since the
date of execution and delivery of the Third Supplemental Indenture, and not
heretofore specifically mortgaged and pledged under the Indenture:

    The Northeast Quarter of Section Fifteen, Township Forty-six North of 
Range Sixty Three West of the Sixth Principal Meridian, Weston county, 
Wyoming.

    TOGETHER with, all and singular, the tenements, hereditaments and 
appurtenances belonging or in any wise appertaining to said property or any
part thereof, with the reversion and reversions, remainder and remainders,
tolls, rents, revenues, issues, earnings, income, products and profits
thereof, and all the estate, right, title and interest and claim whatsoever,
at law as well as in equity, which the Company now has or may hereafter
acquire in and to said property, rights and franchises, and every part and
parcel thereof;

    TO HAVE AND TO HOLD all such properties, real, personal and mixed, 
mortgaged, pledged or conveyed by the Company as aforesaid, or intended so to 
be, unto the Trustee and its successors and assigns forever;

    IN TRUST, NEVERTHELESS, for the same purposes and upon the same trusts, 
terms and conditions, and subject to and with the same provisos and covenants 
as are set forth in the Indenture (as amended by this Supplemental Indenture), 
with the same effect in all respects as if the property and rights herein 
described and herein conveyed to the Trustee had, at the time of the execution 
and delivery of the Original Indenture, been owned by the Company and had been
specifically and at length described in and conveyed to the Trustee by the
Original Indenture as a part of the property therein stated to be conveyed.
 
                                 ARTICLE ONE
                            BONDS OF SERIES E, 3%

    SECTION 1.01. There is hereby created a series of Bonds, known as and 
entitled "First Mortgage Bonds, Series E, 3%", due March
<PAGE>
 
                                      5
    
1, 1980 and the form thereof shall be as provided in this Supplemental
Indenture.

    The aggregate principal amount of Bonds of Series E which may be
authenticated and delivered and outstanding under the Indenture and this 
Supplemental Indenture shall be unlimited except as provided in Articles Four, 
Five and Six of the Original Indenture and in Section 4.04 of the Third 
Supplemental Indenture. The aggregate principal amount of Bonds of Series E 
initially authenticated and delivered hereunder shall be $990,000. The Bonds of 
Series E shall bear interest at the rate of 3% per annum until the principal 
thereof becomes due and payable and thereafter, if default be made in the
payment of such principal, at the rate of 6% per annum until the principal
thereof shall be paid, and shall mature March 1, 1980.

    The Bonds of Series E shall be coupon Bonds of the denomination of $1,000
and registered Bonds without coupons of the denominations of $1,000 and any 
multiples of $1,000, which may be executed by the Company and delivered to the 
Trustee for authentication and delivery. The coupon Bonds of Series E shall be 
dated March 1, 1950, which date shall be the date of the commencement of the 
first interest period for all Bonds of Series E, and shall be numbered from EM1 
consecutively upwards. The registered Bonds of Series E without coupons shall 
be dated as provided in Section 2.05 of the Original Indenture, and shall be 
numbered, in the case of Bonds of the denomination of $1,000, from ERM1 
consecutively upwards and, in the case of Bonds of other denominations, from 
ERX1 consecutively upwards. All Bonds of Series E shall bear interest from
their respective dates, such interest to be payable semi-annually on the first
day of March and September in each year, the first interest payment date being 
September 1, 1950. The principal of, premium, if any, and interest on the Bonds
of Series E shall be payable at the principal office of the Trustee, in the
Borough of Manhattan, The City of New York, in any coin or currency of the
United States of America which at the time of payment shall be legal tender
for the payment of public and private debts. The Bonds of Series E shall be
subject to redemption as provided in Section 1.03 and Article Two of this
Supplemental Indenture. 
<PAGE>
 
                                      6
    
    The Trustee is hereby appointed Registrar in respect of the Bonds
of Series E, and the principal office of the Trustee in the Borough of
Manhattan, The City of New York, is hereby designated as the office or agency 
of the Company in said Borough where notices or demands in respect of Bonds 
of Series E, or their appurtenant coupons, may be served.

    The definitive Bonds of Series E shall be issued in the form of engraved 
Bonds or Bonds printed or lithographed on steel engraved borders. Subject to
the foregoing provisions of this Section and to the provisions of Section 2.11
of the Original Indenture, all definitive Bonds of Series E shall be fully 
interchangeable for other Bonds of the same series, and, upon surrender to the 
Trustee at its principal office, shall be exchangeable for other Bonds of the
same series in coupon form or fully registered form and in such authorized
denomination or denominations in the same aggregate principal amount, as may he
requested by the holder surrendering the same. The Company will execute, and
the Trustee shall authenticate and deliver, coupon Bonds and/or registered
Bonds without coupons, whenever the same shall be required for any such
exchange.

     SECTION 1.02. The texts of the Bonds of Series E, the coupons to be 
attached thereto and the certificate of authentication of the Trustee to be
executed thereon, are to be substantially in the following forms, respectively:

                      (FORM OF COUPON BOND OF SERIES E)

No. EM .....                                                           $1,000

                    BLACK HILLS POWER AND LIGHT COMPANY

             FIRST MORTGAGE BOND SERIES E, 3%, DUE MARCH 1, 1980


    BLACK HILLS POWER AND LIGHT COMPANY (hereinafter called the
"Company"), a corporation organized and existing under the laws of
the State of South Dakota, for value received, hereby promises to pay
to bearer, or, if this Bond be registered as to principal, then to the
<PAGE>
 
                                      7
 
registered owner hereof, on the 1st day of March, 1980, at the principal office
of the Trustee, in the Borough of Manhattan, The City of New York, ONE
THOUSAND DOLLARS, in any coin or currency of the United States of America
which at the time of payment shall be legal tender for the payment of public
and private debts, and to pay interest thereon from March 1, 1950, at the rate
of three per cent. per annum, payable at said principal office of the Trustee
in like coin or currency semi-annually on March 1 and September 1 in each year
until the principal hereof shall have become due and payable, and thereafter
if default be made in the payment of such principal, at the rate of six per
cent. per annum until the principal hereof shall be paid. The interest accrued
up to the date of maturity shall be paid only upon presentation and surrender
and according to the tenor of the interest coupons hereto annexed as they
severally mature.

     This Bond is one of an authorized issue of Bonds of the Company known as
its "First Mortgage Bonds", issued and to be issued in one or more series
under, and all equally and ratably secured (except as any sinking,
amortization, improvement, renewal or other analogous fund, established in
accordance with the provisions of the Indenture hereinafter mentioned, may
afford additional security for the Bonds of any particular series) by, an
Indenture of Mortgage and Deed of Trust dated as of September 1, 1941 as
supplemented and amended by Supplemental Indentures dated as of July 15, 1945,
January 15, 1948, January 15, 1949 and March 1, 1950 (hereinafter collectively
called the "Indenture"), executed by the Company to Central Hanover Bank and
Trust Company (herein called the "Trustee"), as Trustee, to which
Indenture and all indentures supplemental thereto reference is hereby made for
a description of the properties mortgaged and pledged, the nature and extent
of the security, the rights of the holders of said Bonds and coupons and of
the Trustee and of the company in respect of such security, and the terms and
conditions upon which said Bonds are and are to be issued and secured.

    To the extent permitted by the Indenture and as provided therein, with the 
consent of the Company and upon the written consent or affirmative vote of at 
least sixty-six and two-thirds per cent. in principal amount of the Bonds then 
outstanding and entitled to consent, and of not less than sixty-six and 
two-thirds per cent. in principal 
<PAGE>
 
                               8
    
amount of the Bonds then outstanding and entitled to consent of each series 
affected thereby in case one or more but less than all of the series of Bonds 
issued under the Indenture are so affected, the rights and obligations of the 
Company and of the holders of Bonds and coupons, and the terms and provisions 
of the Indenture and of any instrument supplemental thereto may be modified 
from time to time, provided that no such modification or alteration shall be
made which would postpone the date fixed herein or in the coupons or in the
Indenture for the payment of the principal of, or any installment of interest
on, the Bonds, or reduce the principal of, or the rate of interest payable on,
the Bonds, or reduce the percentage of the principal amount of Bonds the
consent of which is required for the authorization of any such modification or
alteration, or which would modify, without the written consent of the Trustee,
the rights, duties or immunities of the Trustee.

    As provided in said Indenture, said Bonds are issuable in series which may 
vary as in said Indenture provided or permitted. This Bond is one of a series
of Bonds entitled "First Mortgage Bonds, Series E, 3%", due March 1, 1980.

    This Bond is subject to redemption at the option of the Company at any time 
upon at least thirty (30) days' and not more than fifty (50) days' notice
published in an authorized newspaper in the Borough of Manhattan, The city of
New York, and/or mailed, all as more fully provided in the Indenture, at the
percentages of the principal amount hereinafter set forth and referred to as
the General Redemption prices, together, in each case, with accrued interest
to the redemption date.

    This Bond is entitled to the benefits of a sinking fund as provided in the 
Indenture and is subject to redemption by application of sinking fund moneys
and also out of moneys deposited with the Trustee in certain cases of the
release of properties from the lien of the Indenture, all subject to the
conditions and as more fully set forth in the Indenture, at any time upon the
notice hereinabove mentioned at the percentages of the principal amount
hereinafter set forth and referred to as the Special Redemption prices,
together, in each case, with accrued interest to the redemption date. 
<PAGE>
 
                                      9
    
The General and Special Redemption prices are as follows :

<TABLE>
<CAPTION>
                                             Percentage of principal amount
                                             -------------------------------
                                               General             Special
                                              redemption          redemption
                                                price               price
                                             -----------          ----------
<S>                                          <C>                  <C>
If redeemed on or before
   August 31, 1951.........................     104                 100.99
 
If redeemed during 12 months
   ending August 31,

              1952.........................     103.91              100.96
              1953.........................     103.82              100.94
              1954.........................     103.73              100.92     
              1955.........................     103.63              100.90
              1956.........................     103.53              100.87
              1957.........................     103.43              100.85
              1958.........................     103.33              100.82
              1959.........................     103.22              100.80
              1960.........................     103.11              100.77
              1961.........................     102.99              100.74
              1962.........................     102.88              100.71
              1963.........................     102.76              100.68
              1964.........................     102.63              100.65
              1965.........................     102.51              100.62
              1966.........................     102.37              100.59
              1967.........................     102.24              100.56
              1968.........................     102.10              100.52
              1969.........................     101.96              100.49
              1970.........................     101.81              100.45
              1971.........................     101.66              100.42
              1972.........................     101.51              100.38
              1973.........................     101.35              100.34
              1974.........................     101.19              100.30
              1975.........................     101.02              100.26
              1976.........................     100.84              100.21
              1977.........................     100.67              100.17
              1978.........................     100.48              100.12
              1979.........................     100.30              100.08
If redeemed after
   August 31, 1979.........................     100.10              100.03
</TABLE>

<PAGE>
 
                                      10
    
    If an event of default, as defined in said Indenture, shall occur, the 
principal of this Bond may become or be declared due and payable, in the
manner and with the effect provided in said Indenture.

   This Bond is transferable by delivery unless registered as to principal in
the name of the holder on books of the Company to be kept for such purpose at
the principal office of the Trustee in the Borough of Manhattan, The City of
New York, such registration being noted hereon. After such registration, no
transfer hereof shall be valid unless made upon said books by the registered
owner in person or by attorney authorized in writing and similarly noted
hereon; but this Bond may be discharged from registration by being, in like
manner, transferred to bearer, and thereupon transferability by delivery shall
be restored, but again and from time to time this Bond may be registered or
transferred to bearer as before. Such registration, however, shall not affect
the negotiability of the coupons hereto appertaining, which shall always
continue to be payable to bearer and to be transferable by delivery.

    First Mortgage Bonds, Series E, 3%, due March 1, 1980, are issuable as 
coupon Bonds in the denomination of $1,000 or as fully registered Bonds without 
coupons of the denominations of $1,000 and any multiples of $1,000 which may 
be executed by the Company and delivered to the Trustee for authentication and 
delivery. All Bonds of said series are fully interchangeable for other Bonds of
the same series, and upon surrender to the Trustee at its principal office in
the Borough of Manhattan, The City of New York, are exchangeable for other
Bonds of the same series in coupon form or fully registered form and in such
authorized denomination or denominations in the same aggregate principal
amount, as may be requested by the holder surrendering the same.

    The Company and the Trustee may deem and treat the bearer of this Bond, 
if it be not registered as to principal, or, if this Bond be registered as
herein authorized, the person in whose name the same is registered, and the
bearer of any coupon hereto appertaining, as the absolute owner for the
purpose of receiving payment, and neither the Company nor the Trustee shall be
affected by any notice to the contrary.

    No recourse shall be had for the payment of the principal of or the 
interest on this Bond, or for any claim based hereon or otherwise 
<PAGE>
 
                                      11
    
in respect hereof or of said Indenture or of any indenture supplemental
thereto, against any incorporator, stockholder, director or officer, as such,
past, present or future, of the Company or of any predecessor or successor
corporation, either directly or through the Company or any predecessor or
successor corporation, whether by virtue of any constitution, statute or rule
of law, or by the enforcement of any assessment or penalty or by any legal or
equitable proceeding or otherwise howsoever; all such liability being, by the
acceptance hereof and as a part of the consideration for the issuance hereof,
expressly waived and released by every holder hereof, as more fully provided
in said Indenture; provided, however, that nothing herein or in said Indenture
contained shall be taken to prevent recourse to and the enforcement of the
liability, if any, of any shareholder or any stockholder or subscriber to
capital stock upon or in respect of shares of capital stock not fully paid up.

    Neither this Bond nor any of the annexed interest coupons shall be valid or 
become obligatory for any purpose until the certificate of authentication
hereon shall have been signed by the Trustee, or its successor as Trustee,
under said Indenture.

    IN WITNESS WHEREOF, the Company has caused this Bond to be signed in 
its name by its President or one of its Vice-Presidents, and its corporate seal
to be impressed or imprinted hereon and attested by its Secretary or one of
its Assistant Secretaries, and coupons for said interest, bearing the
facsimile signature of its Treasurer, to be hereunto attached.

    Dated March 1, 1950.
 
                         BLACK HILLS POWER AND LIGHT COMPANY,
                             
                           By
                                                                     President.
   
Attest:

                         Secretary. 
<PAGE>
 
                                      12
 
           (FORM OF INTEREST COUPONS FOR COUPON BONDS OF SERIES E)
 
    On the first day of     , 19  (unless the Bond hereinafter described shall
have been called for previous redemption and payment duly provided therefor),
upon surrender of this coupon, Black Hills Power and Light Company will pay to 
bearer at the principal office of Central Hanover Bank and Trust company, in
the Borough of Manhattan, The City of New York, Fifteen Dollars ($15.00), in
coin or currency of the United States of America which at the time of payment
shall be legal tender for the payment of public and private debts, being six
months interest then due on its First Mortgage Bond, Series E, 3%, due March
1, 1980, No.     .

                                                                     Treasurer.
       
                    (FORM OF REGISTERED BOND OF SERIES E)
  
No. ER . . .                                                          $ . . . 

                     BLACK HILLS POWER AND LIGHT COMPANY
 
             FIRST MORTGAGE BOND, SERIES E, 3%, DUE MARCH 1, 1980
 
    BLACK HILLS POWER AND LIGHT COMPANY (hereinafter called the 
"Company"), a corporation organized and existing under the laws of the State
of South Dakota, for value received, hereby promises to pay to       ,
or registered assigns, on the 1st day of March, 1980, at the principal office
of the Trustee, in the Borough of Manhattan, The City of New York, 
                DOLLARS, in any coin or currency of the United States of 
America which at the time of payment shall be legal tender for the payment of
public and private debts, and to pay interest thereon from the date hereof, at
the rate of three per cent. per annum, payable at said principal office of the
Trustee in like coin or currency semi-annually on March 1 and September 1 in
each year until the principal hereof shall have become due and payable, and
thereafter if default be made in the payment of such principal, at the rate of
six per cent. per annum until the principal hereof shall be paid. 
<PAGE>
 
                                      13
    
     This Bond is one of an authorized issue of Bonds of the Company known as 
its "First Mortgage Bonds", issued and to be issued in one or more series
under, and all equally and ratably secured (except as any sinking,
amortization, improvement, renewal or other analogous fund, established in
accordance with the provisions of the Indenture hereinafter mentioned, may
afford additional security for the Bonds of any particular series) by, an
Indenture of Mortgage and Deed of Trust dated as of September 1, 1941 as
supplemented and amended by Supplemental Indentures dated as of July 15, 1945,
January 15, 1948, January 15, 1949 and March 1, 1950 (hereinafter collectively
called the "Indenture"), executed by the Company to Central Hanover Bank and
Trust Company (herein called the "Trustee"), as Trustee, to which Indenture
and all indentures supplemental thereto reference is hereby made for a
description of the properties mortgaged and pledged, the nature and extent of
the security, the rights of the holders of said Bonds and the coupons
appurtenant to coupon Bonds and of the Trustee and of the Company in respect
of such security, and the terms and conditions upon which said Bonds are and
are to be issued and secured.

     To the extent permitted by the Indenture and as provided therein, with the 
consent of the company and upon the written consent or affirmative vote of at 
least sixty-six and two-thirds per cent. in principal amount of the Bonds then 
outstanding and entitled to consent, and of not less than sixty-six and
two-thirds per cent. in principal amount of the Bonds then outstanding and
entitled to consent of each series affected thereby in case one or more but
less than all of the series of Bonds issued under the Indenture are so
affected, the rights and obligations of the Company and of the holders of
Bonds and coupons appurtenant to coupon Bonds, and the terms and provisions of
the Indenture and of any instrument supplemental thereto may be modified from
time to time, provided that no such modification or alteration shall be made
which would postpone the date fixed herein or in the coupons or in the
Indenture for the payment of the principal of, or any installment of interest
on, the Bonds, or reduce the principal of, or the rate of interest payable on,
the Bonds, or reduce the percentage of the principal amount of Bonds the
consent of which is required for the authorization of any such modification or
alteration, or which would modify, without the written consent of the Trustee,
the rights, duties or immunities of the Trustee.
<PAGE>
 
                                      14
    
     As provided in said Indenture, said Bonds are issuable in series which may 
vary as in said Indenture provided or permitted. This Bond is one of a series
of Bonds entitled "First Mortgage Bonds, Series E, 3%", due March 1,
1980.

     This Bond is subject to redemption, in whole or in part, at the option of
the Company at any time, upon at least thirty (30) days' and not more than
fifty (50) days' notice published in an authorized newspaper in the Borough of
Manhattan, The City of New York, and/or mailed, all as more fully provided in
the Indenture, at the percentages of the principal amount hereinafter set
forth and referred to as the General Redemption prices, together, in each
case, with accrued interest to the redemption date.

     This Bond is entitled to the benefits of a sinking fund as provided in the 
Indenture and is subject to redemption in whole or in part by application of 
sinking fund moneys and also out of moneys deposited with the Trustee in 
certain cases of the release of properties from the lien of the Indenture, all 
subject to the conditions and as more fully set forth in the Indenture, at any
time upon the notice hereinabove mentioned at the percentages of the principal 
amount hereinafter set forth and referred to as the Special Redemption prices, 
together, in each case, with accrued interest to the redemption date.

    The General and Special Redemption prices are as follows:
 
<TABLE>
<CAPTION>
                                                Percentage of principal amount
                                              ---------------------------------
                                                General               Special
                                              redemption             redemption
                                                price                  price
                                              -----------            -----------
<S>                                           <C>                    <C>
If redeemed on or before
    August 31, 1951 .......................     104                    100.99
If redeemed during 12 months
    ending August 31,
              1952.........................     103.91                 100.96
              1953.........................     103.82                 100.94
              1954 ........................     103.73                 100.92
              1955.........................     103.63                 100.90
              1956.........................     103.53                 100.87
              1957 . ......................     103.43                 100.85
              1958.........................     103.33                 100.82
</TABLE>
<PAGE>
 
                                      15
<TABLE>
<S>                                           <C>                    <C>
              1959.........................     103.22                 100.80
              1960.........................     103.11                 100.77
              1961.........................     102.99                 100.74
              1962.........................     102.88                 100.71
              1963.........................     102.76                 100.68
              1964.........................     102.63                 100.65
              1965.........................     102.51                 100.62
              1966.........................     102.37                 100.59
              1967.........................     102.24                 100.56
              1968.........................     102.10                 100.52
              1969.........................     101.96                 100.49
              1970 ........................     101.81                 100.45
              1971 ........................     101.66                 100.42
              1972.........................     101.51                 100.38
              1973.........................     101.35                 100.34
              1974.........................     101.19                 100.30
              1975.........................     101.02                 100.26
              1976.........................     100.84                 100.21
              1977.........................     100.67                 100.17
              1978.........................     100.48                 100.12
              1979.........................     100.30                 100.08
If redeemed after
    August 31, 1979 .......................     100.10                 100.03
</TABLE>
 
     If this Bond or any portion thereof ($1,000 or a multiple) shall be duly 
called for redemption as provided in said Indenture, this Bond or such portion
thereof shall (unless the Company shall default in the payment of the
redemption price) cease to bear interest from and after the date fixed for
redemption.

    Upon any partial redemption of this Bond, this Bond may, at the option of
the registered holder hereof be either (a) surrendered to the Trustee in
exchange for one or more new Bonds of this series, in either registered or
coupon form (but only in authorized denominations), for the principal amount
of the unredeemed portion of this Bond or (b) submitted to the Trustee for
notation hereon by the Trustee of the payment of the portion of the principal
hereof so called for redemption.

    If an event of default, as defined in said Indenture, shall occur, the 
principal of this Bond may become or be declared due and payable, in the
manner and with the effect provided in said Indenture.
<PAGE>
 
                                      16
    
    This Bond is transferable by the registered owner hereof in person or by 
attorney authorized in writing, at said principal office of the Trustee, upon 
surrender for cancellation of this Bond and on payment of charges, and upon any 
such transfer a new registered Bond without coupons, of the same series, for
the same aggregate principal amount, will be issued to the transferee in
exchange herefor. First Mortgage Bonds, Series E, 3%, due March 1, 1980, are
issuable as coupon Bonds in the denomination of $1,000 or as fully registered
Bonds without coupons of the denominations of $1,000 and any multiples of
$1,000 which may be executed by the Company and delivered to the Trustee for
authentication and delivery. All Bonds of said series are fully
interchangeable for other Bonds of the same series, and upon surrender to the
Trustee at its principal office in the Borough of Manhattan, The City of New
York, are exchangeable for other Bonds of the same series in coupon form or
fully registered form and in such authorized denomination or denominations in
the same aggregate principal amount, as may be requested by the holder
surrendering the same.


    The Company and the Trustee may deem and treat the persona in whose 
name this Bond is registered as the absolute owner hereof, for the purpose of 
receiving payment of or on account of the principal hereof and interest due 
hereon, and neither the Company nor the Trustee shall be affected by any notice 
to the contrary.

    No recourse shall be had for the payment of the principal of or the 
interest on this Bond, or for any claim based hereon or otherwise in respect
hereof or of said Indenture or of any indenture supplemental thereto, against
any incorporator, stockholder, director or officer, as such, past, present or
future, of the Company or of any predecessor or successor corporation, either
directly or through the Company or any predecessor or successor corporation,
whether by virtue of any constitution, statute or rule of law, or by the
enforcement of any assessment or penalty or by any legal or equitable
proceeding or otherwise howsoever; all such liability being, by the acceptance
hereof and as a part of the consideration for the issuance hereof, expressly
waived and released by every holder hereof, as more fully provided in said
Indenture; provided, however, that nothing herein or in said Indenture
contained shall be taken to prevent recourse to and the enforcement of 
<PAGE>
 
                                      17
    
the liability, if any, of any shareholder or any stockholder or subscriber to
capital stock upon or in respect of shares of capital stock not fully paid up.

    This Bond shall not be valid or become obligatory for any purpose until the 
certificate of authentication hereon shall have been signed by the Trustee, or
its successor as Trustee, under said Indenture.

    IN WITNESS WHEREOF, the Company has caused this Bond to be signed in its 
name by its President or one of its Vice-Presidents, and its corporate seal to
be impressed or imprinted hereon and attested by its Secretary or one of its
Assistant Secretaries.

    Dated,
                
                               BLACK HILLS POWER AND LIGHT COMPANY,
 
                                     By
    
Attest:                                                             President.

                                     Secretary.

             (FORM OF TRUSTEE'S CERTIFICATE FOR BOTH COUPON AND
                        REGISTERED BONDS OF SERIES E )

 
    This is one of the Bonds described in the within mentioned Indenture.

                                       CENTRAL HANOVER BANK AND TRUST COMPANY,
                                                    As Trustee,
   
                                           By
                                                        Authorized Officer.
      
<PAGE>
 
                                      18
    
    SECTION 1.03. The Bonds of Series E shall be redeemable (except as 
otherwise provided in Section 2.02 of this Supplemental Indenture) at the
option of the Company, at any time and from time to time, in whole or in part,
in the manner and upon the notice provided in Article Ten of the Original
Indenture at general redemption prices constituting, respectively, percentages
of the principal amount thereof, as follows:

    104.   if redeemed on or before August 31, 1951;
 
    103.91 if redeemed thereafter and on or before August 31, 1952;
    103.82 if redeemed thereafter and on or before August 31, 1953;
    103.73 if redeemed thereafter and on or before August 31, 1954;
    103.63 if redeemed thereafter and on or before August 31, 1955; 
    103.53 if redeemed thereafter and on or before August 31, 1956;
    103.43 if redeemed thereafter and on or before August 31, 1957;
    103.33 if redeemed thereafter and on or before August 31, 1958;
    103.22 if redeemed thereafter and on or before August 31, 1959; 
    103.11 if redeemed thereafter and on or before August 31, 1960; 
    102.99 if redeemed thereafter and on or before August 31, 1961; 
    102.88 if redeemed thereafter and on or before August 31, 1962; 
    102.76 if redeemed thereafter and on or before August 31, 1963; 
    102.63 if redeemed thereafter and on or before August 31, 1964; 
    102.51 if redeemed thereafter and on or before August 31, 1965; 
    102.37 if redeemed thereafter and on or before August 31, 1966; 
    102.24 if redeemed thereafter and on or before August 31, 1967; 
    102.10 if redeemed thereafter and on or before August 31, 1968; 
    101.96 if redeemed thereafter and on or before August 31, 1969; 
    101.81 if redeemed thereafter and on or before August 31, 1970; 
    101.66 if redeemed thereafter and on or before August 31, 1971; 
    101.51 if redeemed thereafter and on or before August 31, 1972; 
    101.35 if redeemed thereafter and on or before August 31, 1973; 
    101.19 if redeemed thereafter and on or before August 31, 1974; 
    101.02 if redeemed thereafter and on or before August 31, 1975; 
    100.84 if redeemed thereafter and on or before August 31, 1976;
    100.67 if redeemed thereafter and on or before August 31, 1977; 
    100.48 if redeemed thereafter and on or before August 31, 1978; 
    100.30 if redeemed thereafter and on or before August 31, 1979; 
    100.10 if redeemed thereafter and prior to maturity;
 
together, in each case, with accrued interest to the redemption date.
<PAGE>
 
                                      19
    
    Any moneys applied to the redemption of Bonds of Series E pursuant to the 
provisions of Section 8.08(b) of the Original Indenture shall be so applied at
the special redemption price stated in Section 2.02 of this Supplemental 
Indenture.


                                 ARTICLE TWO

                      SINKING FUND FOR BONDS OF SERIES E
  
    SECTION 2.01. The Company will, subject to the provisions of this Article, 
maintain a sinking fund (herein called the "Series E Sinking Fund") to be 
applied as hereinafter provided for the Bonds of Series E, and for that purpose 
will pay to the Trustee on the 15th day of July in each year so long as any of
the Bonds of said series shall be outstanding, commencing with the year 1951
(said dates being herein sometimes referred to as "Series E Sinking Fund
Payment Dates") a sum in cash equal to 1/2 of 1% of the total principal amount
of Bonds of Series E, theretofore at any time authenticated and delivered
under Article Four, Five or Six of the Original Indenture, but in no case
less than $5000; provided that the Company shall have the right to satisfy any
obligation in respect of the Series E Sinking Fund in whole or in part by
delivering to the Trustee not less than 30 days prior to any Series E Sinking
Fund Payment Date, any Bonds of Series E, theretofore authenticated and
delivered hereunder and not previously canceled or called for redemption,
together with (in the case of coupon bonds) all unmatured coupons thereto
appertaining, and the Trustee shall credit such Series E Sinking Fund
obligation with an amount equal to the principal amount of the Bonds of Series
E, so delivered.

    If the Company elects to satisfy any Series E Sinking Fund obligation in 
whole or in part by the delivery of Bonds of Series E, as above provided, it
shall deliver such Bonds of said series, together with (in the case of coupon
bonds) all unmatured coupons thereto appertaining not less than 30 days prior
to any Series E Sinking Fund Payment Date, and also written notice of such
election, signed by the Treasurer or an Assistant Treasurer of the Company,
specifying the aggregate principal amount of Bonds of said series so delivered
and the distinctive numbers thereof and specifying the amount, if any, of such
Series E Sinking Fund obligation to be satisfied by the payment 
<PAGE>
 
                                      20
    
of cash; and in case such notice be not given and such Bonds of Series E be not 
delivered within the time so required, the Series E Sinking Fund obligation
shall be paid entirely in cash.

    SECTION 2.02. Whenever the Trustee shall be required to redeem Bonds 
pursuant to the provisions of this Article, the Company shall in each case pay
to the Trustee in cash the redemption premium thereon and all interest accrued
on such Bonds to the date of redemption thereof. All cash paid to the Trustee
in satisfaction of the requirements of the Series E Sinking Fund shall be
applied by the Trustee before the first day of September next ensuing its
receipt to the redemption of Bonds of Series E.


    Whenever any Bonds of said series are to be called for redemption under 
the provisions of this Section 2.02 of this Supplemental Indenture, the Trustee 
shall proceed to select for redemption, from the Bonds of said series, in the 
manner provided in Article Ten of the Original Indenture, a principal amount of 
Bonds of said series equal to the amount of cash in said Series E Sinking Fund 
which is to be applied to the redemption of Bonds of Series E, and for and on 
behalf of the Company and in the name of the Company, the Trustee shall give 
notice, as required by the provisions of Article Ten of the Original Indenture,
of the redemption for the Series E Sinking Fund of the Bonds so selected.
Subject to the provisions of this Article, the redemption of such Bonds shall
be effected in the manner and upon the terms provided in Section 10.03 of the
Original Indenture at special redemption prices constituting, respectively,
percentages of the principal amount thereof, as follows:

    100.99 if redeemed on or before August 31, 1951;
    100.96 if redeemed thereafter and on or before August 31, 1952;
    100.94 if redeemed thereafter and on or before August 31, 1953;
    100.92 if redeemed thereafter and on or before August 31, 1954;
    100.90 if redeemed thereafter and on or before August 31, 1955;
    100.87 if redeemed thereafter and on or before August 31, 1956;
    100.85 if redeemed thereafter and on or before August 31, 1957;
    100.82 if redeemed thereafter and on or before August 31, 1958;
    100.80 if redeemed thereafter and on or before August 31, 1959;
    100.77 if redeemed thereafter and on or before August 31, 1960;
    100.74 if redeemed thereafter and on or before August 31, 1961; 
<PAGE>
 
                                      21
     
    100.71 if redeemed thereafter and on or before August 31, 1962;
    100.68 if redeemed thereafter and on or before August 31, 1963;
    100.65 if redeemed thereafter and on or before August 31, 1964;
    100.62 if redeemed thereafter and on or before August 31, 1965;
    100.59 if redeemed thereafter and on or before August 31, 1966;
    100.56 if redeemed thereafter and on or before August 31, 1967;
    100.52 if redeemed thereafter and on or before August 31, 1968;
    100.49 if redeemed thereafter and on or before August 31, 1969;
    100.45 if redeemed thereafter and on or before August 31, 1970;
    100.42 if redeemed thereafter and on or before August 31, 1971;
    100.38 if redeemed thereafter and on or before August 31, 1972;
    100.34 if redeemed thereafter and on or before August 31, 1973;
    100.30 if redeemed thereafter and on or before August 31, 1974;
    100.26 if redeemed thereafter and on or before August 31, 1975;
    100.21 if redeemed thereafter and on or before August 31, 1976;
    100.17 if redeemed thereafter and on or before August 31, 1977;
    100.12 if redeemed thereafter and on or before August 31, 1978;
    100.08 if redeemed thereafter and on or before August 31, 1979;
    100.03 if redeemed thereafter and prior to maturity;

together, in each case, with accrued interest to the redemption date.

                                ARTICLE THREE

                     AMENDMENTS TO THE ORIGINAL INDENTURE

 
    SECTION 3.01. Paragraph F of Section 5.01 of Article Five of the
Original Indenture as heretofore amended by the Third Supplemental
Indenture is hereby further amended by adding thereto the following:
 
    ``and provided further, however, that, in case the Bonds of
    Series E first authenticated and delivered are authenticated and
    delivered pursuant to this Article in May, 1950, there may be
    substituted for said Net Earnings Certificate (in connection with
    the authentication and delivery of said bonds) a similar certificate
    covering the twelve (12) months period ended October 31, 1949
    certified by an Accountant plus a similar certificate covering the
    twelve (12) months period ended March 31, 1950 not certified
    by an Accountant but certified by the Treasurer of the Company,
    each such certificate to show Certifiable Net Earnings of the Com-
<PAGE>
 
                                      22
        
    pany as set forth therein to have been at least equal to two (2) times the 
    aggregate amount of the annual Interest Charges on the Secured Bonded 
    Debt of the Company as set forth therein and that the Net Earnings of the 
    Electric Properties as set forth therein is not less than eighty-five per
    cent. (85%) of the required minimum amount of Certifiable Net Earnings."
    
    SECTION 3.02. Section 9.16 of Article Nine of the Original Indenture as 
heretofore amended by the Third Supplemental Indenture is hereby further 
amended by deleting the words "Bonds of Series A, Series B, Series C or Series 
D" therein and substituting therefore the words "Bonds of Series A, Series B, 
Series C, Series D or Series E".
    
    
     SECTION 3.03. Section 9.22 of Article Nine of the Original Indenture as 
heretofore amended by the Third Supplemental Indenture is hereby further
amended by deleting the words "Bonds of Series A, Series B, Series C or 
Series D" therein and substituting therefore the words "Bonds of Series A, 
Series B, Series C, Series D or Series E".
 
                                ARTICLE FOURTH
 
                           MISCELLANEOUS PROVISIONS
        
      
    SECTION 4.01. So long as any Bonds of Series A, Series B, Series C, 
Series D or Series E shall be outstanding, no Bonds shall be authenticated and 
delivered under Article Six of the Original Indenture and no Trust Moneys shall 
be paid over under Article Eight of the Original Indenture upon the basis of
the redemption or retirement of Bonds of Series A for the Series A Improvement 
Fund provided for in Article Eleven of the Original Indenture, upon the basis
of the redemption or retirement of Bonds of Series B for the Series B
Sinking Fund provided for in Article Two of the First Supplemental Indenture,
upon the basis of the redemption or retirement of Bonds of Series C for the
Series C Sinking Fund provided for in Article Two of the Second Supplemental
Indenture, upon the basis of the redemption or retirement of Bonds of Series D
for the Series D Sinking Fund provided for in Article Two of the Third
Supplemental Indenture or upon the basis of the redemption or retirement of
Bonds of Series E for the Series E Sinking Fund provided for in Article Two of
this Supplemental Indenture. All moneys at any time constituting a part of
said Series A
<PAGE>
 
                                      23
    
Improvement Fund, Series B Sinking Fund, Series C Sinking Fund, Series D 
Sinking Fund or Series E Sinking Fund shall be deemed to be Bonded Cash.

    SECTION 4.02. The Company covenants that so long as any Bonds of 
Series E shall remain outstanding it will comply with the covenants contained 
in Sections 9.06, 9.15 and 9.20 of the Original Indenture.


    SECTION 4.03. The aggregate principal amount of Bonds which, 
immediately after the execution and delivery of this Supplemental Indenture,
will be outstanding under the provisions of, and secured by, the Indenture, as 
amended by this Supplemental Indenture, will be $7,638,000, consisting of 
$1,980,000 aggregate principal amount of First Mortgage Bonds, Series A,
3 3/8%, due September 1, 1971, $2,073,000 aggregate principal amount of First
Mortgage  Bonds, Series B, 3%, due July 15, 1975, $1,095,000 aggregate
principal amount of First Mortgage Bonds, Series C, 3 3/8%, due July 15, 1975,
$1,500,000 aggregate principal amount of First Mortgage Bonds, Series D,
3 3/8%, due January 15, 1979 and $990,000 aggregate principal amount of First
Mortgage  Bonds, Series E, 3%, due March 1, 1980.


    SECTION 4.04. The Company, by the execution hereof, acknowledges 
that true copies of this and the First, Second and Third Supplemental
Indentures have been delivered to and received by it.


    SECTION 4.05. Except as heretofore amended and as amended by this 
Supplemental Indenture all the provisions, terms and conditions of the Original 
Indenture shall continue in full force and effect.


    SECTION 4.06. This Supplemental Indenture may be executed in several 
counterparts, all or any of which may be treated for all purposes as one
original and shall constitute and be one and the same instrument.


    IN WITNESS WHEREOF, BLACK HILLS POWER AND LIGHT Company, party hereto of 
the first part, has caused this Supplemental indenture to be executed on its
behalf by its President or one of its Vice-Presidents and its corporate seal
to be hereto affixed and to be attested be its Secretary or an Assistant
Secretary, and CENTRAL HANOVER BANK
<PAGE>
 
                                      24
    
AND TRUST COMPANY, party hereto of the second part, in evidence of its
acceptance of thee trust hereby created, has caused this Supplemental
Indenture to be executed on its behalf by one of its Vice-Presidents or
Assistant Vice-Presidents and its corporate seal to be hereto affixed and to
be attested by its Secretary or an Assistant Secretary, all as of the day
and year first above written.

                              BLACK HILLS POWER AND LIGHT COMPANY,
   
(CORPORATE SEAL)                          J. B. FRENCH
                                                   President.
Attest:
      B. B. NEEL
          Secretary.
 
Signed sealed and delivered by
   BLACK HILLS POWER AND LIGHT COMPANY
     in the presence of:
 
       EVELYN E. DIDIER
       WALLACE H. LERE
 
                             CENTRAL HANOVER BANK AND TRUST COMPANY,
 
(CORPORATE SEAL)                          J. T. HARRIGAN
                                                    Vice-President.
Attest:
      F. E. EGLY
          Assistant Secretary.

Signed, sealed and delivered by
   CENTRAL HANOVER BANK AND TRUST COMPANY
     in the presence of:
 
       R. P. McGUIRK
       C. W. ANDERSON
 
    $1,089 of United States Internal Revenue Stamps withy reference to the
Bonds of Series E, initially issued hereunder have been duly affixed to a
counterpart of the Original Indenture in the custody of the Trustee and duly
canceled as required by law.
<PAGE>
 
                                      25
 
STATE OF SOUTH DAKOTA 
                       ss.:
COUNTY OF PENNINGTON 


    On this the 1st day of May, 1950, before me, ETTA NEWKIRK, the 
undersigned officer, personally appeared J. B. FRENCH, to me personally 
known, who acknowledged himself to be, and being by me duly sworn, did say 
that he is the President of BLACK HILLS POWER AND LIGHT COMPANY, a 
corporation, and that the seal affixed to the foregoing instrument is the
corporate  seal of said corporation and that said instrument was executed by,
and signed in  the name of, the corporation, by him, as such President and
sealed in behalf of the corporation by authority of its Board of Directors for
the purposes therein contained, and the said J. B. FRENCH acknowledged the
same as the free act and deed of said corporation.

    IN WITNESS WHEREOF, I hereunto set my hand and official seal.
 
                                                 ETTA NEWKIRK
                                                       Notary Public
My commission expires Jan. 23, 1952.                         (NOTARIAL SEAL)
 
STATE OF NEW YORK
                    ss.:
COUNTY OF NEW YORK 

    On this the 3rd day of May, 1950, before me, THOMAS F. MEARGHER, the 
undersigned officer, personally appeared J. T. HARRIGAN, to me personally
known, who acknowledged himself to be, and being by me duly sworn, did say
that he is the Vice-President of CENTRAL HANOVER BANK AND TRUST COMPANY, a
corporation, and that the seal affixed to the foregoing instrument is the
corporate seal of said corporation and that said instrument was executed by,
and signed in the name of, the corporation, by him, as such Vice-President and
sealed in behalf of the corporation by authority of its Board of Trustees for 
the purposes therein contained, and the said J. T. HARRIGAN acknowledged the 
same as the free act and deed of said corporation. 

    IN WITNESS WHEREOF, I hereunto set my and and official seal.
 
                                                 THOMAS F. MEAGHER
                                                       Notary Public 
My commission expires March 30, 1952.
                                             THOMAS F. MEAGHER
                                  Notary Public of the State of New York
                                               No. 41-7878600
(NOTARIAL SEAL)
                                          Qualified Queens County
                                Ctfs. filed with Queens, New York County Clks.
                                      Queens, New York Register's Offices
                                          Term Expires March 30, 1952


         

         

<PAGE>
 
                                                               [Conformed Copy]
================================================================================




                     BLACK HILLS POWER AND LIGHT COMPANY

                                      TO
 
                     MANUFACTURERS HANOVER TRUST COMPANY,
                                                      As Trustee



                                  ----------

                      Nineteenth Supplemental Indenture
                         Dated as of February 15, 1977
  
                                  ----------



                  Supplemental to Indenture of Mortgage and
                 Deed of Trust Dated as of September 1, 1941
   




================================================================================
<PAGE>
 
    NINETEENTH SUPPLEMENTAL INDENTURE, dated as of the 15th day of February,
1977, between BLACK HILLS POWER AND LIGHT COMPANY, a corporation organized and
existing under the laws of the State of South Dakota (hereinafter called the
``Company''), party of the first part, and MANUFACTURERS HANOVER TRUST COMPANY,
a corporation organized and existing under the laws of the State of New York,
as Trustee under the Indenture hereinafter mentioned (hereinafter called the
``Trustee''), party of the second part.

    WHEREAS, in order to secure an authorized issue of First Mortgage Bonds of
the Company, the Company has executed and delivered an Indenture of Mortgage
and Deed of Trust to Central Hanover Bank and Trust Company (subsequently
known as The Hanover Bank), as Trustee, dated September 1, 1941, hereinafter
referred to as the ``Original Indenture'', and has also executed and delivered
to said Trustee and to Manufacturers Hanover Trust Company (which on September
8, 1961 became the Trustee under the Original Indenture, as theretofore
supplemented and amended, by virtue of the merger of said The Hanover Bank
into Manufacturers Trust Company under said name Manufacturers Hanover Trust
Company), as Trustee, various Supplemental Indentures supplementing and/or
modifying the Original Indenture, which Supplemental Indentures are therein
and herein sometimes referred to, and are respectively dated, as follows:

<TABLE>
<S>                                       <C>
    First Supplemental Indenture .......... July 15, 1945
    Second Supplemental Indenture ......... January 15, 1948
    Third Supplemental Indenture .......... January 15, 1949
    Fourth Supplemental Indenture ......... March 1, 1950
    Fifth Supplemental Indenture .......... March 1, 1952
    Sixth Supplemental Indenture .......... July 1, 1956
    Seventh Supplemental Indenture ........ May 1, 1957
    Eighth Supplemental Indenture ......... May 1, 1959
    Ninth Supplemental Indenture .......... April 1, 1960

</TABLE>
 
<PAGE>
 
                                       2

<TABLE>

<S>                                      <C>
    Tenth Supplemental Indenture .......... August 1, 1960
    Eleventh Supplemental Indenture ....... June 1, 1961
    Twelfth Supplemental Indenture ........ October 1, 1962
    Thirteenth Supplemental Indenture ..... May 1, 1963
    Fourteenth Supplemental Indenture ..... June 1,1969
    Fifteenth Supplemental Indenture ...... June 15, 1974
    Sixteenth Supplemental Indenture ...... August 1, 1974
    Seventeenth Supplemental Indenture .... July 15, 1975
    Eighteenth Supplemental Indenture ..... May 1, 1976

</TABLE>

and the Company has also executed and delivered to said The Hanover Bank, as 
Trustee, Subordination Agreements dated December 29, 1950 and September 19, 
1955, also supplementing the Original Indenture which, as supplemented and 
amended by said eighteen Supplemental Indentures and said Subordination 
Agreements, is hereinafter referred to as the ``Indenture''; and

    WHEREAS, pursuant to the provisions of the Indenture, First Mortgage Bonds,
Series D, 3 3/8%, due January 15, 1979, Series E, 3%, due March 1, 1980, Series
F, 3 7/8%, due March 1, 1982, Series G, 4%, due July 1, 1986, Series H, 5%, due
May 15, 1987, Series I, 4 7/8%, due May 1, 1989, Series J, 5 1/2%, due April 1,
1990, Series K, 5 3/8%, due August 1, 1990, Series L, 5 1/8%, due June 1, 1991,
Series M, 4 3/4%, due October 1, 1992, Series N, 4 1/2%, due May 1, 1993, Series
O, 7.80%, due June 1, 1999, Series P, 10.50%, due August 1, 2004 and Series Q,
11%, due July 15, 1985 have been duly issued and all or part of the Bonds of
each of said Series are presently outstanding and secured by the Indenture and
are hereinafter referred to collectively as the ``Bonds''; and

    WHEREAS, the Company deems it advisable that the Indenture be amended as
herein provided and the holders of the requisite amount of the outstanding
Bonds issued under the Indenture have duly consented, pursuant to the
provisions of Section 18.11 of the Indenture, to the
<PAGE>
 
                                       3


execution of this Supplemental Indenture and have lodged with the Trustee
instruments duly setting forth such consent; and

    WHEREAS, the Company, in the exercise of the powers and authority
conferred upon and reserved to it under and by virtue of the provisions of the 
Original Indenture, and particularly the provisions contained in Articles 
Seventeen and Eighteen thereof, and pursuant to appropriate resolutions of its 
Board of Directors, has duly resolved and determined to make, execute and 
deliver to the Trustee a Supplemental Indenture in the form hereof for the 
purposes herein provided; and

    WHEREAS, all conditions and requirements necessary to make this
Supplemental Indenture a valid, binding and legal instrument in accordance with 
its terms have been done, performed and fulfilled, and the execution and
delivery hereof have been in all respects duly authorized:

    NOW, THEREFORE, THIS INDENTURE WITNESSETH: That the Company, in
consideration of the premises and of one dollar to it duly paid by the Trustee
at or before the ensealing and delivery of these presents, the receipt whereof
is hereby acknowledged, and of other good and valuable consideration, in order
to amend certain provisions of the Indenture and the Bonds, hereby further 
covenants and agrees to and with the Trustee and its successors in the trust 
under the Indenture for the benefit of all those who shall from time to time
hold the Bonds as follows:

                                 ARTICLE ONE
 
                  AMENDMENTS TO THE INDENTURE AND THE BONDS
 
    SECTION 1.01. Paragraph D of Section 1.01 of Article One of the Original
Indenture is hereby amended by inserting the words ``the Chairman of the Board
or'' before the words ``the President'' wherever the latter words appear in
said Paragraph D.

    SECTION 1.02. Section 2.07 of Article Two of the Original Indenture is
hereby amended by inserting the words ``its Chairman of the Board or'' before 
the words ``its President'' where the latter words appear in said Section 2.07.
<PAGE>
 
                                       4

    SECTION 1.03. Paragraph O of Section 4.01 of Article Four of the Original
Indenture as amended by Section 3.04 of Article Three of the Third Supplemental
Indenture dated as of January 15, 1949 is hereby amended so as to read as
follows:

         ``O. The `Certifiable Net Earnings' of the Company for any particular
    period shall be computed and ascertained by deducting from the total of the
    gross operating revenues of the Company for such period the following:
    
         All operating expenses and other proper charges (other than those
    charged to capital accounts or surplus) including (a) all Federal, state 
    and local taxes (other than taxes in respect of income or profits and other
    taxes imposed on or measured by income or profits); (b) rentals, insurance,
    current repairs and maintenance; and (c) provision for depreciation,
    obsolescence, depletion and property renewals and replacements in an amount
    not less than the greater of the following items (i) and (ii)
     
              (i) the amount actually deducted on the books of the Company in
         respect thereof; or
       
              (ii) an amount equal to the amount of the excess of (a) 15% of
         the Gross Utility Operating Revenues of the Company during such period,
         after deducting from such Gross Utility Operating Revenues a sum equal
         to the cost to the Company of electric energy purchased by the Company
         during such period for resale, over (b) actual expenditures for
         maintenance and repairs during such period;
    
    but not including any interest to the extent that such interest constitutes 
    Interest Charges on Long-Term Debt as defined in Paragraph F of Section
    4.02.

         The Gross Operating Revenues of the Company shall consist of Gross
    Utility Operating Revenues of the Company, plus the Net Non-Operating Income
    of the Company. The term 'Gross Utility Operating Revenues' of the Company
    shall mean the aggregate gross operating revenues derived from the operation
    of the utility properties owned or leased by the Company. The term `Net
    Non-Operating Income' of the Company shall
<PAGE>
 
                                       5

    mean net income derived from but not necessarily limited to the following: 
    (a) merchandising, jobbing and contract work; (b) rental of non-utility 
    properties; (c) interest and dividend income including dividends from 
    Subsidiaries; (d) allowance for funds used during construction; and
    (e) other miscellaneous non-operating income; provided, however, that
    profits or losses resulting from the sale, abandonment or other disposi-
    tion of capital assets or securities of the Company and the Company's
    equity in the undistributed earnings of Subsidiaries, shall not be taken
    into account in the calculation of Net Non-Operating Income.

         If any of the property of the Company shall have been owned by it
    during a part, but not during the whole, of any period for which such
    computation is to be made, or shall have been acquired by the Company after
    the expiration of said period, and shall be subject to the direct lien of
    this Indenture, the net earnings of such property during such part of such
    period as shall have preceded the acquisition thereof by the Company shall
    be included in the Gross Utility Operating Revenues for the purposes of this
    Paragraph, but such net earnings shall be ascertained and computed by the
    methods hereinabove in this Paragraph prescribed. In case, within or after
    the particular period for which the computation is made, (i) the Company
    shall have obtained the release of any property pursuant to the provisions
    of Section 7.02 of a Fair Value in excess of $100,000 in any one case as
    shown by the Certificate of the Company provided for in Paragraph B or C
    of Section 7.02, or (ii) any property shall have been taken by the exercise
    of the power of eminent domain or purchased in the manner specified in
    Section 7.03 and the award for any such taking or the proceeds of any such
    purchase shall have exceeded $100,000 in any one case, then and in any such
    case the net earnings. estimated if necessary, of such property for the
    whole of such period shall be excluded from the Certifiable Net Earnings for
    the purposes of this Paragraph.

         Subject to the foregoing provisions of this Section, all
    determinations of earnings pursuant to this Indenture shall be made, and all
    balance sheets and other financial statements to be
<PAGE>
 
                                       6

    delivered hereunder shall be prepared, in accordance with the practice 
    prescribed by any regulatory authority having jurisdiction over the Company
    or other lawfully prescribed practice or, in the absence of any practice
    prescribed by law, in accordance with sound accounting practice and, where
    consistent with such practice and with the foregoing provisions of this
    Section, on the same basis as that used in preparing the financial
    statements included in the annual report of the Company for the preceding
    fiscal year.''
   
    SECTION 1.04. Paragraph F of Section 4.02 of Article Four of the Original
Indenture is hereby amended so as to read as follows:

         ``F. A NET EARNINGS CERTIFICATE of the Company, complying with the
    provisions of Section 1.02, dated not more than 30 days prior to the
    application for the authentication and delivery of such Bonds, certified by
    an Accountant, and setting forth:
    
              (1) The amount of the Certifiable Net Earnings of the Company,
         for a period of 12 consecutive calendar months within the 15 calendar 
         months immediately preceding the date on which the application for the
         authentication and delivery of the Bonds is made, and stating 
         separately the Gross Utility Operating Revenues and the Net 
         Non-Operating Income and the operating expenses of the Company and
         other deductions from such Gross Utility Operating Revenues and Net
         Non-Operating Income pursuant to Paragraph O of Section 4.01, with the
         principal subdivisions thereof.
     
              (2) The aggregate amount of the annual `Interest Charges on
         Long-Term Debt' of the Company, which term shall mean the annual 
         interest charges on
     
                   (a) all Bonds outstanding hereunder at the date of said
              Certificate, provided, however, that in the ease of any Bonds 
              which shall at such time be pledged as security for any
              indebtedness of the Company, the amount of the annual interest
              charges on such pledged Bonds shall be deemed to be either the
              amount of the annual interest charges on such indebtedness or the
              amount of the annual interest charges on such pledged Bonds,
              whichever shall be greater; and
<PAGE>
 
                                       7


                   (b) all Bonds the authentication and delivery of which is
              applied for in such application and in any other pending
              application; and
   
                   (c) all indebtedness secured by a lien upon the Trust
              Estate, or any part thereof, prior to the lien of this Indenture,
              other than a Prepaid Lien; and
   
                   (d) all other Funded Indebtedness as defined in Section
              4.04 of the Seventeenth Supplemental Indenture hereto dated as of
              July 15, 1975 as amended by Section 2.01 of the Eighteenth
              Supplemental Indenture hereto dated as of May 1, 1976, other than
              any such Funded Indebtedness which at such time shall be secured
              by the pledge of any Bonds;
  
         provided, however, that there shall be excluded from such computation 
         the annual interest charges on any Bonds or indebtedness which is to 
         be paid, redeemed or otherwise retired, or provision for the retirement
         of which is to be made, so that the same will cease to be outstanding
         prior to or concurrently with the authentication and delivery of the
         Bonds then applied for.
 
              (3) That the amount of the Certifiable Net Earnings of the
         Company set forth as provided by Clause (1) of this Paragraph have 
         been at least equal to two (2) times the aggregate amount of the 
         annual Interest Charges on Long-Term Debt of the Company, set forth as
         provided by Clause (2) of this Paragraph.

              (4) That such Certifiable Net Earnings have been computed and
         ascertained as provided in Paragraph O of Section 4.01.

         The Company covenants and agrees that if the annual Interest Charges
    on the Long-Term Debt of the Company shall be increased after the date of
    the Earnings Certificate hereinabove in this Paragraph described, and before
    the authentication and delivery of the Bonds then applied for, the Company
    will file with the Trustee a new Earnings Certificate showing the amount
<PAGE>
 
                                       8

    of said annual Interest Charges on the Long-Term Debt as so increased
    - it being the intention hereof that no Bonds shall be authenticated and
    delivered under the provisions of this Article, unless the ratio provided
    for by Clause (3) of this Paragraph shall have been established with
    respect to the aggregate amount of the annual Interest Charges on the Long-
    Term Debt of the Company as constituted at the time of the authentication
    and delivery of the Bonds then applied for; but the Trustee shall, subject
    to the provisions of Section 15.02, be entitled to assume, in the absence
    of such new Earnings Certificate, that the aggregate amount of the annual
    Interest Charges on the Long-Term Debt of the Company, as constituted at
    the time of the authentication and delivery of the Bonds then applied for,
    are as stated in the Earnings Certificate filed with the Trustee as
    aforesaid.
 
         The Earnings Certificate provided for in this Paragraph shall be
    certified by an Independent Public Accountant selected by the Company and
    approved by the Trustee, in the exercise of reasonable care, if, but only
    if, the aggregate principal amount of the Bonds to be authenticated and
    delivered on the basis thereof and of other Bonds authenticated and 
    delivered since the commencement of the then current calendar year (other
    than those with respect to which an Earnings Certificate is not required or
    with respect to which an Earnings Certificate verified by an Independent
    Public Accountant has previously been furnished) is 10% or more of the
    aggregate principal amount of Bonds at the time outstanding.''

    SECTION 1.05. From and after February 1, 1977 all Bonds of Series D,
Series E, Series F, Series G, Series H, Series I, Series J, Series K, Series L, 
Series M, Series N, Series O, Series P, and Series Q shall bear interest at the 
increased rate per annum hereinafter set forth opposite the original interest
rate for each of such Series. Wherever in the Bonds of each such Series or in
the Supplemental Indenture creating Bonds of each such Series reference is made
to the rate of interest on the Bonds of such Series, the respective Bonds and
Supplemental Indenture are hereby amended to substitute the increased interest
rate in each and every place where the original interest rate for such Series
shall occur; provided, however, that such amendment shall become effective on
February 1, 1977 and in no event prior thereto.
<PAGE>
 
                                       9


    The amendment set forth in this Section applies to each of the
Supplemental Indentures set forth below:

<TABLE>
 
<CAPTION>

       Supplemental Indenture       Bonds       Original        Increased
            Dated as of           of Series   Interest Rate   Interest Rate
       ----------------------     ---------   -------------   -------------
<S>    <C>                       <C>         <C>             <C> 

Third       -January 15, 1949         D          3 3/8%           3 5/8%
Fourth      -March 1, 1950            E          3    %           3 1/4%
Fifth       -March 1, 1952            F          3 7/8%           4 1/8%
Sixth       -July 1, 1956             G          4    %           4 1/4%
Seventh     -May l, 1957              H          5    %           5 1/4%
Eighth      -May 1, 1959              I          4 7/8%           5 1/8%
Ninth       -April 1, 1960            J          5 1/2%           5 3/4%
Tenth       -August 1, 1960           K          5 3/8%           5 5/8%
Eleventh    -June 1, 1961             L          5 1/8%           5 3/8%
Twelfth     -October 1, 1962          M          4 3/4%           5    %
Thirteenth  -May 1, 1963              N          4 1/2%           4 3/4%
Fourteenth  -June 1, 1969             O           7.80%            8.05%
Sixteenth   -August 1, 1974           P          10.50%           10.75%
Seventeenth -July 15, 1975            Q          11   %           11.25%

</TABLE>

    SECTION 1.06. Each Bond of Series D, Series E, Series F, Series G, Series
H, Series I, Series J, Series K, Series L, Series M, Series N, Series O, Series
P, or Series Q hereafter issued in exchange for or in place of any thereof
theretofore issued shall (unless textually revised as hereinafter provided) be
stamped or otherwise marked with a notation in the following form (the
appropriate blanks to be filled in so as to indicate for the Bonds of each such
Series the pertinent Supplemental Indenture, Series letter, increased interest 
rate and amended designation):
 
         ``Pursuant to a certain Supplemental Indenture, dated as of February
    15, 1977, modifying and amending the Original Indenture, dated as of
    September 1, 1941, and the Supplemental Indenture, dated as of          ,
        pursuant to which this Bond of Series     was issued, the rate of
    interest payable on this Bond
<PAGE>
 
                                       10

    of Series   from and after February 1, 1977, has been increased to    
    per annum, and the designation of this Bond has been changed to `First
    Mortgage Bonds, Series   ,          '. For a complete statement of the said
    modifications and amendments effected by the said Supplemental Indenture, an
    executed counterpart of which is on file at the principal office of
    Manufacturers Hanover Trust Company, as Trustee, reference thereto is hereby
    made.''

    Any outstanding Bond of Series D, Series E, Series F, Series G, Series H,
Series I, Series J, Series K, Series L, Series M, Series N, Series O, Series
P or Series Q shall, if the holder thereof so demands, be stamped or otherwise
marked with a notation as aforesaid by or under the supervision of the Trustee
upon presentation thereof for the purpose at the principal office of the
Trustee.
  
    If the Company or the Trustee shall so determine, new Bonds so textually
revised as, in the opinion of the Trustee and the Board of Directors of the 
Company, to conform to the amendments of the Indenture and the Bonds provided
for in this Supplemental Indenture, shall be executed, authenticated and 
delivered, and, upon the demand of the holders of any of the Bonds then 
outstanding, shall be issued, without cost to such holders, in exchange for such
outstanding Bonds upon surrender of such Bonds with all unmatured coupons 
appertaining thereto.


                                  ARTICLE TWO

                            MISCELLANEOUS PROVISIONS

    SECTION 2.01. The Company, by the execution hereof, acknowledges that a
true copy of this Supplemental Indenture has been delivered to and received by
it.

    SECTION 2.02. Except as heretofore amended and as amended by this
Supplemental Indenture, all the provisions, terms and conditions of the Original
Indenture shall continue in full force and effect.
  
    SECTION 2.03. This Supplemental Indenture may be executed in several
counterparts, all or any of which may be treated for all purposes as one
original and shall constitute and be one and the same instrument.
<PAGE>
 
                                       11

    IN WITNESS WHEREOF, BLACK HILLS POWER AND LIGHT COMPANY, party hereto of
the first part, has caused this Supplemental Indenture to be executed on its
behalf by the Chairman of the Board or its President or one of its Vice
Presidents and its corporate seal to be hereto affixed and to be attested by its
Secretary or an Assistant Secretary and MANUFACTURERS HANOVER TRUST COMPANY,
party hereto of the second part, in evidence of its acceptance of the trust
hereby created, has caused this Supplemental Indenture to be executed on its
behalf by one of its Vice Presidents or Assistant Vice Presidents and its
corporate seal to be hereto affixed and to be attested by its Secretary or an
Assistant Secretary, all as of the day and year first above written.

                                       BLACK HILLS POWER AND LIGHT
                                         COMPANY,
[CORPORATE SEAL]
                                            By ROBERT G. ASHEIM
                                                           President.
Attest: 

  GEORGE T. LOCKE
              Secretary.
           
Signed, sealed and delivered by
   Black Hills Power and Light
   Company in the presence of:
      LOUISE S. KELLEY
      GEORGE D. WEEDMAN

                                       MANUFACTURERS HANOVER TRUST
                                         COMPANY,
[CORPORATE SEAL]
                                            By E. F. COCKINGS
                                              Assistant Vice President.
Attest: 

  JAMES M. FOLEY
            Assistant Secretary.
          
Signed, sealed and delivered by
   Manufacturers Hanover Trust
   Company in the presence of:
      D. A. URSITTI, JR.
      W. B. DODGE
<PAGE>
 
                                       12


STATE OF SOUTH DAKOTA 
                       ss.:
COUNTY OF PENNINGTON 

    On this 15th day of February, 1977, before me, MARY S. KELLER, the
undersigned officer, personally appeared ROBERT G. ASHEIM, to me personally
known, who acknowledged himself to be, and being by me duly sworn, did say that
he is President of BLACK HILLS POWER AND LIGHT COMPANY, a corporation, and that
the seal affixed to the foregoing instrument is the corporate seal of said
corporation and that said instrument was executed by, and signed in the name
of, said corporation, by him, as such President and sealed in behalf of said
corporation by authority of its Board of Directors for the purposes therein
contained, and the said ROBERT G. ASHEIM acknowledged the same as the free act
and deed of said corporation.

    IN WITNESS WHEREOF, I hereunto set my hand and official seal.
  
[NOTARIAL SEAL]                          MARY S. KELLER
                                         Notary Public
                               My commission expires June 27, 1979

STATE OF NEW YORK
                       ss.:
COUNTY OF NEW YORK

    On this 28th day of February, 1977, before me, MARY LEONARDI, the
undersigned officer, personally appeared E. F. COCKINGS, to me personally known,
who acknowledged himself to be, and being by me duly sworn, did say that he is
Assistant Vice President of MANUFACTURERS HANOVER TRUST COMPANY, a corporation,
and that the seal affixed to the foregoing instrument is the corporate seal of
said corporation and that said instrument was executed by, and signed in the
name of, said corporation, by him, as such Assistant Vice President and sealed
in behalf of said corporation by its Board of Directors for the purposes therein
contained, and the said E. F. COCKINGS acknowledged the same as the free act and
deed of said corporation.

    IN WITNESS WHEREOF, I hereunto set my hand and official seal.

[NOTARIAL SEAL]                          MARY LEONARDI
                                         Notary Public
                                         MARY LEONARDI
                                Notary Public, State of New York
                                        No. 40-7495520
                                  Qualified in Putnam County
                              Certificate Filed in New York County
                                Commission Expires March 30, 1978

<PAGE>
 
                                                            [CONFORMED COPY]
==============================================================================


                     BLACK HILLS POWER AND LIGHT COMPANY

                                      TO

 
                     MANUFACTURERS HANOVER TRUST COMPANY,



                                                                   As Trustee


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

                       TWENTIETH SUPPLEMENTAL INDENTURE
                           Dated as of April 1,1977
 
                             --------------------
 
              Supplemental to Indenture of Mortgage and Deed of
                     Trust Dated as of September 1, 1941
 
 
 
===============================================================================
<PAGE>
 
    TWENTIETH SUPPLEMENTAL INDENTURE, dated as of the 1st day 
of April, 1977 between BLACK HILLS POWER AND LIGHT COMPANY a 
corporation organized and existing under the laws of the State of South 
Dakota (herein called the "Company"), party of the first part and 
MANUFACTURERS HANOVER TRUST COMPANY, a corporation organized and existing under
the laws of the State of New York, as Trustee under the Indenture hereinafter 
mentioned (hereinafter called the "Trustee''), party of the second part.

    WHEREAS, in order to secure an authorized issue of First Mortgage 
Bonds of the Company, the Company has executed and delivered an Indenture 
of Mortgage and Deed of Trust to Central Hanover Bank and Trust Company 
(subsequently known as The Hanover Bank), as Trustee, dated September 1, 
1941, hereinafter referred to as the "Original Indenture'', and has also
executed and delivered to said Trustee and to Manufacturers Hanover Trust
Company (which on September 8, 1961 became the Trustee under the Original
Indenture, as theretofore supplemented and amended, by virtue of the merger
of said The Hanover Bank into Manufacturers Trust Company, under said name 
Manufacturers Hanover Trust Company), as Trustee, various Supplemental 
Indentures supplementing and/or modifying the Original Indenture, respectively 
dated as of July 15, 1945, January 15, 1948, January 15, 1949, March 1, 1950, 
March 1, 1952, July 1, 1956, May 1, 1957, May l, 1959, April 1, 1960, August 1, 
1960, June 1, 1961, October 1, 1962, May 1, 1963, June 1, 1969, June 15, 1974,
August 1, 1974, July 15, 1975, May 1, 1976 and February 15, 1977 and the 
Company has also executed and delivered to said The Hanover Bank, as Trustee, 
Subordination Agreements dated December 29, 1950 and September 19, 1955, also 
supplementing the Original Indenture which, as supplemented and amended by 
said nineteen Supplemental Indentures and said Subordination Agreements, is 
hereinafter referred to as the "Indenture''; and


    WHEREAS, pursuant to the provisions of the Indenture, First Mortgage 
Bonds have been duly issued and are presently outstanding and secured by the 
Indenture as follows: Series D, 3 5/8%, due January 15, 1979, Series E, 3 1/4%, 
due March 1, 1980, Series F, 4 1/8%, due March 1, 1982, Series G, 4 1/4%, due 
July 1, 1986, Series H, 5 1/4%, due May 15, 1987, Series I, 5 1/8%, due May 1, 
1989, Series J, 5 3/4%, due April 1, 1990, Series K, 5 5/8% due August 1, 
1990, Series L, 5 3/8%, due June 1,
<PAGE>
 
1991, Series M, 5%, due October 1, 1992, Series N, 4 3/4%, due May 1, 1993, 
Series O, 8.05%, due June 1, 1999, Series P, 10.75%, due August 1, 2004 and 
Series Q, 11 1/4%, due July 15, 1985; and

    WHEREAS, the Company has entered into an Agreement dated as of April 
1, 1977 (the "Lawrence Agreement No. 1") with Lawrence County, South 
Dakota ("Lawrence") a political subdivision of the State of South Dakota, for
the acquisition, construction, installation and equipping of certain
facilities for air and water pollution control purposes at the Kirk Electric
Generating Plant of the Company and an Agreement dated as of April 1, 1977
(the "Pennington Agreement No. 1") with Pennington County, South Dakota
("Pennington") also a political subdivision of the State of South Dakota,
for the acquisition, construction, installation and equipping of certain
facilities for air and water pollution control purposes at the Ben French
Electric Generating Plant of the Company and said pollution control facilities
will be financed through the issuance by each County of its respective
Pollution Control Revenue Bonds (Black Hills Power and Light Company Project)
Collateralized 1977 Series A (hereinafter respectively, the "Lawrence Series
A Bonds" and the "Pennington Series A Bonds" and, collectively, the
"County Series A Bonds") and in connection therewith the Company will
execute and deliver its First Mortgage Bonds of Series R and Series S
provided for and created by this Supplemental Indenture which will be pledged
by Lawrence and Pennington, respectively, to the trustee (the "County
Trustee") under each County's Indenture of Trust No. 1 dated as of April 1,
1977 (hereinafter, respectively, the "Lawrence Pollution Control Indenture"
and the "Pennington Pollution Control Indenture" and, collectively, the
"Pollution Control Indentures") providing for the issuance of the related
County's Series A Bonds with respect to said pollution control facilities, as
security for the payment of the principal of and premium, if any, and interest 
on the related Series A Bonds; and


    WHEREAS, the Company has entered into a further agreement dated as of 
April 1, 1977 (the "Lawrence Agreement No. 2") with Lawrence, for the 
acquisition, construction, installation and equipping of certain additional
facilities at the Kirk Electric Generating Plant of the Company and a further
agreement dated as of April 1, 1977 (the "Pennington Agreement No. 2") with
Pennington, for the acquisition, 
<PAGE>
 
                                      3

 
construction, installation and equipping of certain additional facilities
at the Ben French Electric Generating Plant of the Company and said
facilities will be financed through the issuance by each County of its
respective Industrial Development Revenue Bonds (Black Hills Power
and Light Company Project) Collateralized 1977 Series B (hereinafter
respectively, the "Lawrence Series B Bonds" and the "Pennington
Series B Bonds" and, collectively, the "County Series B Bonds")
and in connection therewith the Company will execute and deliver
its First Mortgage Bonds of Series T and Series U provided for and
created by this Supplemental Indenture which will be pledged by
Lawrence and Pennington, respectively, to the County Trustee under
the related County's Indenture of Trust No. 2 dated as of April 1, 1977
(hereinafter, respectively, the "Lawrence Revenue Bond Indenture"
and the "Pennington Revenue Bond Indenture" and, collectively, the
"Revenue Bond Indentures") providing for the issuance of the related
County's Series B Bonds with respect to said facilities, as security for
the payment of the principal of and premium, if any, and interest on
the related Series B Bonds; and
 
    WHEREAS, as permitted by the Indenture, the Company by resolutions of 
its Board of Directors duly adopted, has determined to create a new series of 
bonds to be known as its "First Mortgage Bonds, Series R" (hereinafter 
sometimes called the "Bonds of Series R"), "First Mortgage Bonds, Series S" 
(hereinafter sometimes called the "Bonds of Series S"), "First Mortgage
Bonds, Series T" (hereinafter sometimes called the "Bonds of Series T") and
"First Mortgage Bonds, Series U" (hereinafter sometimes called the "Bonds
of Series U"), each in the form, having the characteristics and being
entitled to the benefits as in this Supplemental Indenture provided; and
 
    WHEREAS, the Company, in the exercise of the powers and authority conferred
upon and reserved to it under and by virtue of the provisions of the Original
Indenture, and particularly the provisions contained in Articles Two and 
Seventeen thereof, and pursuant to appropriate resolutions of its Board of 
Directors, has duly resolved and determined to make, execute and deliver to the 
Trustee a Supplemental Indenture in the form hereof for the purposes herein 
provided; and
<PAGE>
 
                                      4

    WHEREAS, the Company is about to issue, under the Indenture, as hereby 
amended and supplemented, its Bonds of Series R in the principal amount of 
$5,850,000 in fully registered form and without coupons and its Bonds of Series 
S in the principal amount of $2,050,000 in fully registered form and without 
coupons payable to the County Trustee, as assignee of Lawrence and Pennington
under the related Pollution Control Indenture. Said Bonds of Series R and 
Series S will otherwise be non-transferable except as required to effect (i) a 
transfer after an event of default under the related Pollution Control
Indenture in the course of the exercise of rights and remedies consequent upon
such event of default, or (ii) a transfer to a successor trustee under the
related Pollution Control Indenture; and

     WHEREAS, the Company is about to issue, under the Indenture, as hereby 
amended and supplemented, its Bonds of Series T in the principal amount of 
$1,000,000 in fully registered form and without coupons and its Bonds of Series 
U in the principal amount of $1,000,000 in fully registered form and without 
coupons payable to the County Trustee, as assignee of Lawrence and Pennington
under the related Revenue Bond Indenture. Said Bonds of Series T and Series
U will otherwise be non-transferable except as required to effect (i) a 
transfer after an event of default under the related Revenue Bond Indenture in 
the course of the exercise of rights and remedies consequent upon such event of 
default, or (ii) a transfer to a successor trustee under the related Revenue
Bond Indenture; and

    WHEREAS, all conditions and requirements necessary to make this Supplemental
Indenture a valid, binding and legal instrument in accordance with its terms
have been done, performed and fulfilled, and the execution and delivery hereof
have been in all respects duly authorized:

    NOW, THEREFORE, THIS INDENTURE WITNESSETH: That the Company in consideration
of the premises and of one dollar to it duly paid by the Trustee at or before
the ensealing and delivery of these presents, the receipt whereof is hereby
acknowledged, and of other good and valuable consideration, in order to better
secure the payment both of the principal of and interest on all Bonds issued
under the Indenture and that may be issued under this or any other indentures 
<PAGE>
 
                                      5

 
supplemental thereto, according to their tenor and effect, and the performance 
by the Company of all the Covenants and conditions herein and therein 
contained, and in order to establish the terms of the Bonds of Series R, Series 
S, Series T and Series U and to amend certain provisions of the Indenture, 
hereby further covenants and agrees to and with the Trustee and its successors 
in the trust under the Indenture for the benefit of all those who shall from
time to time hold the Bonds of Series R, Series S, Series T and Series U as
follows:

                                ARTICLE ONE
 
                          BONDS OF SERIES R, 6 5/8%
 
    SECTION 1.01. There is hereby created a series of Bonds, known as and 
entitled "First Mortgage Bonds Series R, 6 5/8%", due April 1, 2007, and the 
form thereof shall be as provided in this Supplemental Indenture.

    SECTION 1.02. The Bonds of Series R shall be evidenced by a single 
registered bond in the principal amount and denomination of Five Million Eight
Hundred Fifty Thousand Dollars ($5,850,000) finally due April 1, 2007 but
payable in installments as follows:

<TABLE>
<CAPTION>
  April 1                  Principal          April 1                Principal
Of the Year                  Amount         Of the Year                Amount
- -----------                ---------        -----------              ---------
<S>                       <C>               <C>                     <C>
   1993                   $500,000.00          2001                 $500,000.00
   1994                    500,000.00          2002                  500,000.00
   1995                    500,000.00          2003                  170,000.00
   1996                    500,000.00          2004                  170,000.00
   1997                    500,000.00          2005                  170,000.00
   1998                    500,000.00          2006                  170,000.00
   1999                    500,000.00          2007                  170,000.00
   2000                    500,000.00

</TABLE>
  
The Bonds of Series R shall bear interest on the unpaid principal balance at
the rate of 6 5/8% per annum payable semi-annually on October l and April 1
commencing October 1, 1977 until due and payable and shall bear interest at
the rate of 6% per annum on any overdue principal and on any overdue
installment of interest (to the extent that payment of such interest is
enforceable under applicable law). April 1, 1977 shall be the date of
commencement of the first
<PAGE>
 
                                      6
 
 
interest period for such Bonds. All such bonds shall be dated as provided in 
Section 2.05 of the Original Indenture.

     The single Bond of Series R shall be lettered R and numbered 1 and shall,
upon issuance, be delivered by the Company to and registered in the name of the 
County Trustee and shall be transferable only as required to effect an 
assignment thereof to a successor County Trustee. Bonds issued upon transfer 
shall be lettered R and numbered from 2 upwards and issued in the same 
$5,850,000 denomination but all payments of principal theretofore made on the 
Bonds of Series R shall be duly noted thereon by the Trustee.

    It is expected that the Company, pursuant to the Lawrence Agreement No. 1,
will furnish directly to the County Trustee at its principal corporate trust
office all funds required for any and all payments of principal of, and interest
and premium on, the Lawrence Series A Bonds (or that in lieu of any such payment
the Company or Lawrence will deposit with the County Trustee Lawrence Series A
Bonds for cancellation) and that corresponding payments of interest and of
installments of principal (including premiums if appropriate) on the Bonds of
Series R will automatically be effected in accordance with the provisions of the
Lawrence Agreement No. 1. Whenever payment or provision therefor has been made
in respect of the principal, premium, if any, or interest on all or any portion
of the Lawrence Series A Bonds in accordance with the Lawrence Pollution Control
Indenture, the corresponding principal installment, premium, if any, or interest
on the Bonds of Series R shall be deemed paid to the extent such payment or
provision therefor has been made. Unless payment then is or has been made as
above provided, payment of the principal of, and premium, if any, and interest
on the single Bond of Series R shall be made to the County Trustee for the
account of Lawrence in funds immediately available at said office of the County
Trustee, in each case on or prior to the due date for such payment. The Trustee
may at any and all times conclusively assume that the obligation of the Company
to make payments with respect to the principal of and premium, if any, and
interest on the Bonds of Series R, so far as such payments at the time have
become due, has been fully satisfied and discharged unless and until the Trustee
shall have received a written notice from the County Trustee signed by one of
its officers stating (i) that timely payment of principal of, or premium or
interest on,
<PAGE>
 
                                      7
 
Bonds of Series R has not been made, (ii) that the Company is in arrears as to 
the payments required to be made by it to the County Trustee pursuant to the 
Lawrence Agreement No. 1 after giving effect to any available moneys in the 
Bond Fund provided by the Lawrence Pollution Control Indenture and (iii) the 
amount of the arrearage.

    The County Trustee, by acceptance of the single Bond of Series R, shall 
agree to make prompt notation thereon of all payments and prepayments on 
account of principal thereof made or occurring under any provisions of the 
Lawrence Agreement No. 1 or of this Supplemental Indenture, and to surrender 
said Bond to the Trustee upon final payment thereof. If and to the extent 
payment of the Lawrence Series A Bonds is deemed to have been made 
pursuant to the terms of the Lawrence Pollution Control Indenture, payment of 
the Bonds of Series R shall be deemed to have been made to the same extent.

    The Trustee is hereby appointed Registrar in respect of the Bonds of Series 
R, and the principal corporate trust office of the Trustee in the Borough of 
Manhattan, The City of New York, is hereby designated as the office or agency 
of the Company in said Borough where notices or demands in respect of Bonds 
of Series R may be served.

    SECTION 1.03. The text of the Bonds of Series R and the certificate of 
authentication of the Trustee to be executed thereon, are to be substantially
in  the following forms, respectively:

No. R .....                                                         $5,850,000
                      
                     BLACK HILLS POWER AND LIGHT COMPANY
 
           FIRST MORTGAGE BOND SERIES R, 6 5/8%, DUE APRIL 1, 2007

    BLACK HILLS POWER AND LIGHT COMPANY (hereinafter called the "Company"),
a corporation organized and existing under the laws of the State of South
Dakota, for value received, hereby promises to pay to NORTHWESTERN NATIONAL
BANK OF MINNEAPOLIS as trustee or its successor in trust (the "County
Trustee"), under an Indenture of Trust No. 1 dated as of April 1, 1977 (the
"Lawrence Pollution Control Indenture") between Lawrence County, South
Dakota and said County Trustee, FIVE MILLION EIGHT HUNDRED FIFTY THOUSAND
DOLLARS, ($5,850,000) payable in installments as follows:
<PAGE>
 
                                      8
<TABLE>
<CAPTION>
 April 1                  Principal            April 1              Principal 
Of the Year                 Amount           Of the Year             Amount
- -----------               ---------          -----------            ---------
<S>                      <C>                 <C>                    <C>
   1993                  $500,000.00            2001                $500,000.00
   1994                   500,000.00            2002                 500,000.00
   1995                   500,000.00            2003                 170,000.00
   1996                   500,000.00            2004                 170,000.00
   1997                   500,000.00            2005                 170,000.00
   1998                   500,000.00            2006                 170,000.00
   1999                   500,000.00            2007                 170,000.00
   2000                   500,000.00
</TABLE>
 
together with interest thereon from April 1, 1977 on the unpaid principal
amount of this Bond at the rate of 6 5/8% per annum until due and payable,
such interest to be paid semi-annually in arrears on October 1 and April 1 in
each year. Overdue principal and overdue installments of interest shall bear
interest at the rate of 6% per annum (to the extent that payment of such
interest is enforceable under applicable law).

    This Bond is one of an authorized issue of Bonds of the Company known as its
"First Mortgage Bonds", issued and to be issued in one or more series under, and
all equally and ratably secured (except as any sinking, amortization,
improvement, renewal or other analogous fund, established in accordance with the
provisions of the Indenture hereinafter mentioned, may afford additional
security for the Bonds of any particular series) by, an Indenture of Mortgage
and Deed of Trust dated as of September l, 1941 as supplemented and amended by
Supplemental Indentures dated as of July 15, 1945, January 15, 1948, January 15,
1949, March 1, 1950, March 1, 1952, July 1, 1956, May 1, 1957, May 1, 1959,
April 1, 1960, August 1,1960, June 1,1961, October 1, 1962, May 1, 1963, June 1,
1969, June 15, 1974, August 1, 1974, July 15, 1975, May 1, 1976 and February 15,
1977 and subordination agreements dated December 29, 1950 and September 19, 1955
(hereinafter collectively called the "Indenture"), executed by the Company to
Manufacturers Hanover Trust Company (herein called the "Trustee"), as Trustee,
to which Indenture and all indentures supplemental thereto reference is hereby
made for a description of the properties mortgaged and pledged, the nature and
extent of the security, the rights of the holders of said Bonds and the coupons
appurtenant to coupon Bonds and of the Trustee and of the Company in respect of
such security, and
<PAGE>
 
                                      9


the terms and conditions upon which said Bonds are and are to be issued and 
secured. 


    To the extent permitted by the Indenture and as provided therein, with the 
consent of the Company and upon the written consent or affirmative vote of at
least sixty-six and two-thirds per cent. in principal amount of the Bonds then
outstanding and entitled to consent, and of not less than sixty-six and
two-thirds per cent. in principal amount of the Bonds then outstanding and
entitled to consent of each series affected thereby in case one or more but
less than all of the series of Bonds issued under the Indenture are so
affected, the rights and obligations of the Company and of the holders of
Bonds and coupons appurtenant to coupon Bonds, and the terms and provisions
of the Indenture and of any instrument supplemental thereto may be modified
from time to time, provided that no such modification or alteration shall be
made which would postpone the date fixed herein or in the coupons or in the
Indenture for the payment of the principal of, or any installment of interest
on, the Bonds, or reduce the principal of, or the rate of interest payable on, 
the Bonds, or reduce the percentage of the principal amount of Bonds the
consent of which is required for the authorization of any such modification or
alteration, or which would modify, without the written consent of the Trustee,
the rights, duties or immunities of the Trustee.

    As provided in said Indenture, said Bonds are issuable in series which may 
vary as in said Indenture provided or permitted. This Bond is one of a series 
of Bonds entitled "First Mortgage Bonds Series R".

     Lawrence County, South Dakota (the "County") is issuing and selling, or
has issued and sold, its Pollution Control Revenue Bonds (Black Hills Power and 
Light Company Project) Collateralized 1977 Series A (hereinafter called the 
"Lawrence Series A Bonds") under and pursuant to the Lawrence Pollution 
Control Indenture. This Bond of Series R is issued as a means of securing 
payment of the purchase price of the Company's purchase from the County of 
certain pollution control facilities pursuant to an Agreement of Sale No. 1 
dated as of April 1, 1977 (the "Lawrence Agreement No. 1"). It is expected 
that the Company will make all payments of principal of, and interest and
premium on, the Lawrence Series A Bonds directly to the County Trustee (or
that in lieu thereof the Company or Lawrence will deposit 
<PAGE>
 
                                      10
 
with the County Trustee Lawrence Series A Bonds for cancellation). All such 
payments shall automatically constitute corresponding payments on the Bonds of 
Series R in accordance with the provisions of the Lawrence Agreement No. 1. 
The holder of this Bond by acceptance hereof agrees that whenever any 
payment or prepayment on account of the principal of this Bond is made or 
occurs under any provision of the Indenture, the Lawrence Agreement No. 1 or 
the Lawrence Pollution Control Indenture, the holder hereof shall promptly note 
on the Schedule of Payments of Principal, the Schedule of Prepayments of 
Principal or the Schedule of Credits of Prepayments of Principal the date and 
amount of each such payment or prepayment of principal, and shall promptly 
notify the Trustee of the amount of each such payment and that the notation of 
payment has been duly made, and further agrees to surrender this Bond to the 
Trustee for cancellation when all principal of, premium, if any, and interest
on this Bond shall have been duly paid. The holder of this Bond further agrees
that if payment of the Lawrence Series A Bonds is deemed to have been made 
pursuant to the provisions of the Lawrence Pollution Control Indenture, payment 
of this Bond shall be deemed to have been made.

    Unless payment then is or has been made pursuant to the foregoing 
paragraph, the principal of and premium, if any, and interest on this Bond
will be paid in lawful money of the United States of America and will be 
payable at the principal corporate trust office of the County Trustee to the 
County Trustee for the account of Lawrence in immediately available funds at 
said office of the County Trustee, in each case on or prior to the due date
for such payment. The holder of this Bond by acceptance hereof agrees that any
such payment of principal, premium or interest on this Bond shall be credited
as and used to make a corresponding payment of principal, premium or interest
on the Lawrence Series A Bonds.

    This Bond is not prepayable prior to April 1, 1987, except that if at any 
time (1) the Company shall elect to prepay installments payable under the 
Lawrence Agreement No. 1 and to cause the Lawrence Series A Bonds to be 
redeemed upon the occurrence of an event specified in Section 9.1 of the 
Lawrence Agreement No. 1 or (2) the Company shall be obligated to prepay 
installments payable under the Lawrence Agreement No. 1 and to cause the 
Lawrence Series A Bonds to be redeemed upon the occurrence of an event 
specified in Section 9.2 of the Lawrence Agreement No. 1, in any such event 
this Bond shall be prepaid by the 
<PAGE>
 
                                      11
 
Company in whole, but not in part (except that upon the occurrence of an event 
specified in paragraph (c) of Section 9.2 of the Lawrence Agreement No. 1 
the Bond shall be prepaid in whole or in part) at 100% of the unpaid principal 
amount thereof plus accrued interest to the prepayment date, which date shall
be the same date as the prepayment date under the Lawrence Agreement No. 1 
determined as set forth in said Sections 9.1 and 9.2. The principal amount of 
Bonds of Series R to be prepaid at any time upon the occurrence of the event 
specified in paragraph (c) of Section 9.2 of the Lawrence Agreement No. 1 shall 
be the same as the principal amount of Lawrence Series A Bonds to be redeemed
as the result of the occurrence of such event.

    This Bond shall be prepaid by the Company on or after April 1, 1987, in 
whole at any time or in part on any interest payment date, if but only if the 
Company shall have elected to prepay installments under the Lawrence 
Agreement No. 1 of like principal amount and to cause Lawrence Series A 
Bonds of like principal amount to be redeemed on the prepayment date in 
accordance with the second paragraph of Section 3.01 of the Lawrence Pollution 
Control Indenture. Such prepayment shall be at the prepayment price determined 
in accordance with the following table plus accrued interest to the prepayment 
date:
 
<TABLE>
<CAPTION>
If prepayment date is during                              
twelve-month-period beginning                              Prepayment
          April 1                                            Price
- -----------------------------                              -----------
<S>                                                        <C>
           1987.............................................. 103    %
           1988.............................................. 102 1/2
           1989.............................................. 102
           1990.............................................. 101 1/2
           1991.............................................. 101
           1992.............................................. 100 1/2
           1993 and thereafter ...............................100
</TABLE>
 
 
    Except to the extent that the Company shall at any time elect, pursuant to 
the provisions of the Lawrence Pollution Control Indenture, to apply as a
credit in respect of a sinking fund obligation thereunder an amount of
Lawrence Series A Bonds redeemed or purchased and delivered to the County
Trustee, any prepayment of only a part of this Bond shall be in inverse order
of the maturities of the several installment payments, e.g., all the portion
of this Bond due on April 1, 2007, must be prepaid before any installment
having an earlier maturity date. In the event that the Company at any time
elects, pursuant to
<PAGE>
 
                                      12
 
 
the provisions of the Lawrence Pollution Control Indenture, to apply as a
credit in respect of a sinking fund obligation thereunder an amount of
Lawrence Series A Bonds redeemed or purchased and delivered to the County
Trustee, the corresponding prepayment of this Bond in the same principal
amount shall be credited against the installment due on this Bond in the same
year as such sinking fund obligation. 


    In each case where this Bond is to be prepaid in whole or in part as 
contemplated herein notice of not less than thirty (30) nor more than sixty
(60) days shall be given by first class mail postage prepaid to the holder of
record of this Bond unless such notice has been waived in writing by the
County Trustee.

    If an event of default as defined in the Indenture shall occur, the
principal of this Bond may become and be declared due and payable in the
manner and with the effect provided in the Indenture. No holder of this Bond
shall have any right to institute any suit or proceeding for the foreclosure
of the Indenture or for any other remedy thereunder, except to the extent and
in the manner set forth in the Indenture.

    This Bond shall be non-transferable except as required to effect (i) a 
transfer to any successor trustee under the Lawrence Pollution Control 
Indenture or (ii) a transfer after an event of default under the Lawrence 
Pollution Control Indenture in the course of the exercise of rights and
remedies consequent upon such event of default. Subject to such restrictions,
this Bond is transferable by the registered holder hereof in person or by
attorney duly authorized in writing, at the office of Manufacturers Hanover
Trust Company, New York, New York, on registry books kept for such purpose at
such office. No charge will be made by the Company for any such transfer of
this Bond. Before any transfer of this Bond by the County Trustee or attorney
duly authorized will be recognized or given effect by the Company or the
Trustee, the County Trustee shall note hereon the date to which interest has
been paid as well as the amounts of all principal payments and prepayments
hereon, and shall notify the Company and the Trustee of the name and address
of the transferee and shall afford the Company and the Trustee the opportunity
of verifying the notation as to payment of interest and principal.

    No recourse shall be had for the payment of the principal of or the
interest on this Bond or for any claim based hereon or otherwise in respect
hereof or of the Indenture or of any agreement supplemental 
<PAGE>
 
                                      13

 
thereto against any subscriber to the capital stock, incorporator or any past, 
present or future stockholder, officer or director of the Company or of any 
predecessor or successor corporation, either directly or through the Company or 
any predecessor or successor corporation or any receiver or trustee in 
bankruptcy, whether by virtue of any constitution or statute or rule of law or
by the enforcement of any assessment or penalty or stock subscription or
otherwise, all such liability, whether at common law or in equity or by
statute or constitution or otherwise, being, to the extent permitted by law,
by the acceptance and as a part of the consideration for the issuance hereof,
expressly waived and released by the registered holder hereof.

     This Bond shall not be valid or become obligatory for any purpose until
the certificate of authentication hereon shall have been signed by the
Trustee, or its successor as Trustee, under said Indenture.

    IN WITNESS WHEREOF, the Company has caused this Bond to be signed in  its
name by its President or one of its Vice-Presidents, and its corporate seal to 
be impressed or imprinted hereon and attested by its Secretary or one of its 
Assistant Secretaries. 


    Dated,
                                         BLACK HILLS POWER AND LIGHT COMPANY, 
 
                                         BY.................................. 
Attest:                                                            President.

......................................
                            Secretary.
 
                       (FORM OF TRUSTEE'S CERTIFICATE)
 
    This is one of the Bonds, of the series designated therein, described in
the within mentioned Indenture.

                                         MANUFACTURER HANOVER TRUST COMPANY, 
                                                                 as Trustee, 
 
                                         BY .................................. 
                                                           Authorized Officer. 
<PAGE>
 
                                      14
 

 
                      SCHEDULE OF PAYMENTS OF PRINCIPAL
<TABLE>
<CAPTION>

 Amount of              Date              Date of          Authorized Official
Installment             Due               Payment               and Title 
- -----------             ----              -------          -------------------
<S>                     <C>               <C>              <C> 
</TABLE> 


 
                     SCHEDULE OF PREPAYMENTS OF PRINCIPAL


<TABLE>
<CAPTION>
Principal Amount                                             Authorized Official
    Prepaid                     Date Prepaid                      and Title
- ----------------                ------------                 ------------------ 
<S>                             <C>                          <C>
</TABLE>
 
 
               SCHEDULE OF CREDITS OF PREPAYMENTS OF PRINCIPAL*


<TABLE>
<CAPTION>

                    Date of Installment 
Prepaid Principal        to which                            Authorized Official
Amount Credited          Credited         Date Credited           and Title
- -----------------   -------------------   -------------      -------------------
<S>                 <C>                   <C>                <C>
</TABLE>

________
    *This Schedule to be completed only in the event that a prepayment is
credited against an installment maturing prior to the then last maturing
unpaid installment. Each prepayment is credited against the then last maturing
unpaid installment unless Lawrence Series A Bonds redeemed or purchased and
delivered to the County Trustee are credited against a sinking fund obligation
under the Lawrence Pollution Control Indenture, whereupon a corresponding 
prepayment in the same principal amount shall be credited against the
installment due in the same year as such sinking fund obligation and entered
in this Schedule.

    (NOTICE: The within Bond may not be transferred until this schedule has
been verified by the Trustee.) 
<PAGE>
 
                                      15



 
    SECTION 1.04. The single Bond of Series R is not prepayable prior to April
1, 1987, except that if, at any time, (1) the Company shall elect to prepay
installments payable under the Lawrence Agreement No. 1 and to cause the
Lawrence Series A Bonds to be redeemed upon the occurrence of an event
specified in Section 9.1 of the Lawrence Agreement No. 1 or (2) the Company
shall be obligated to prepay installments payable under the Lawrence Agreement
No. 1 and to cause the Lawrence Series A Bonds to be redeemed upon the
occurrence of an event specified in Section 9.2 of the Lawrence Agreement No.
1, in any such event the Bonds of Series R shall be prepaid by the Company in
whole, but not in part (except that upon the occurrence of an event specified
in paragraph (c) of Section 9.2 of the Lawrence Agreement No. 1 the Bonds of
Series R shall be prepaid in whole or in part) at 100% of the unpaid principal
amount thereof plus accrued interest to the prepayment date, which date shall
be the same date as the prepayment date under the Lawrence Agreement No. 1
determined as set forth in said Sections 9.1 and 9.2. The principal amount of
Bonds of Series R to be prepaid at any time upon the occurrence of the event
specified in paragraph (c) of Section 9.2 of the Lawrence Agreement No. 1 shall
be the same as the principal amount of Lawrence Series A Bonds to be redeemed
as the result of the occurrence of such event.
 
    The single Bond of Series R shall be prepaid by the Company on or after 
April 1, 1987, in whole at any time or in part on any interest payment date, if
but only if the Company shall have elected to prepay installments under the 
Lawrence Agreement No. I of like principal amount and to cause Lawrence Series
A Bonds of like principal amount to be redeemed on the prepayment date in
accordance with the second paragraph of Section 3.01 of the Lawrence Pollution
Control Indenture. Such prepayment shall be at the prepayment price 
determined in accordance with the following table plus accrued interest to the 
prepayment date:

<TABLE>
<CAPTION>

     If the prepayment date is
     during the twelve months                            Prepayment
        beginning April 1                                  Price
     -------------------------                           ----------
<S>                                                      <C>
 
              1987......................................... 103    % 
              1988......................................... 102 1/2
              1989......................................... 102
              1990......................................... 101 1/2
              1991......................................... 101
              1992......................................... 100 1/2
              1993 and thereafter ......................... 100
</TABLE>
<PAGE>
 
                                      16
 



 
    Except to the extent that the Company shall at any time elect, pursuant to 
the provisions of the Lawrence Pollution Control Indenture, to apply as a
credit in respect of a sinking fund obligation thereunder an amount of
Lawrence Series A Bonds redeemed or purchased and delivered to the County
Trustee, any prepayment of only a part of the single Bond of Series R shall be
in inverse order of the maturities of the several installment payments, e.g.,
all the portion of the single Bond of Series R due on April 1, 2007, must be
prepaid before any installment having an earlier maturity date. In the event
that the Company at any time elects, pursuant to the provisions of the
Lawrence Pollution Control Indenture, to apply as a credit in respect of a
sinking fund obligation thereunder an amount of Lawrence Series A Bonds
redeemed or purchased and delivered to the County Trustee, the corresponding
prepayment of the single Bond of Series R in the same principal amount shall
be credited against the installment due on the single Bond of Series R in the
same year as such sinking fund obligation.


    If Bonds of Series R are to be redeemed in whole or in part as 
provided in this Section 1.04, notice of redemption shall be given by first
class mail, postage prepaid, by or on behalf of the Company, not less than
thirty (30) nor more than sixty (60) days prior to the date of redemption, to
the registered holders of all Bonds so to be redeemed, at their respective
addresses appearing upon the books maintained by the Trustee pursuant to
Section 2.10 of the Original Indenture. Any notice which is mailed as herein
provided shall be conclusively presumed to have been properly and sufficiently
given on the date of such mailing, whether or not the registered holder or
payee, as the case may be, receives the notice. In case of any redemption of
Bonds of such Series by the Trustee pursuant to the provisions of the
Indenture or any indenture supplemental thereto, notice of redemption shall be
given in a similar manner by the Trustee. Notwithstanding any provision of
Article Ten of the Original Indenture, no publication of notice of redemption
of any Bonds of Series R shall be required by the Indenture.

    Except as provided in the immediately preceding paragraph, the provisions 
of Article Ten of the Original Indenture shall in all respects apply to any 
redemption to which this Section 1.04 applies. Nothing in
<PAGE>
 
                                      17
 
 
this Section 1.04 contained shall affect the manner of giving notice of the
redemption of Bonds of the Company of any series other than Bonds of Series R.

    Notwithstanding the provisions of the Original Indenture (including
particularly Section 8.08(b) thereof), the single Bond of Series R is subject
to redemption prior to maturity only upon the events and as set forth in this
Section 1.04. 


                                  ARTICLE TWO
 
                          BONDS OF SERIES S, 6 5/8%
 
 
    SECTION 2.01. There is hereby created a series of Bonds, known as and
entitled "First Mortgage Bonds Series S, 6 5/8%", due April 1, 2007, and the
form thereof shall be as provided in this Supplemental Indenture. 


    SECTION 2.02. The Bonds of Series S shall be evidenced by a single
registered bond in the principal amount and denomination of Two Million Fifty
Thousand Dollars ($2,050,000) finally due April 1, 2007 but payable in
installments as follows: 

<TABLE>
<CAPTION>

     April 1            Principal         April 1              Principal
   Of the Year           Amount         Of the Year              Amount
   -----------          ---------       -----------            --------- 
<S>                     <C>             <C>                   <C>
      1993........... $190,000.00           2001............. $190,000.00
      1994...........  190,000.00           2002.............  190,000.00
      1995...........  190,000.00           2003.............   30,000.00
      1996...........  190,000.00           2004.............   30,000.00
      1997...........  190,000.00           2005.............   30,000.00
      1998...........  190,000.00           2006.............   30,000.00
      1999...........  190,000.00           2007.............   30,000.00
      2000...........  190,000.00
</TABLE>
<PAGE>
 
                                      18
 

    The Bonds of Series S shall bear interest on the unpaid principal balance
at the rate of 6 5/8% per annum payable semi-annually on October 1 and April 1 
commencing October 1, 1977 until due and payable and shall bear interest at the 
rate of 6% per annum on any overdue principal and on any overdue installment 
of interest (to the extent that payment of such interest is enforceable under 
applicable law). April 1, 1977 shall be the date of commencement of the first 
interest period for such Bonds. All such bonds shall be dated as provided in 
Section 2.05 of the Original Indenture.

    The single Bond of Series S shall be lettered S and numbered 1 and shall, 
upon issuance, be delivered by the Company to and registered in the name of the 
County Trustee and shall be transferable only as required to effect an
assignment thereof to a successor County Trustee. Bonds issued upon transfer
shall be lettered S and numbered from 2 upwards and issued in the same
$2,050,000 denomination but all payments of principal theretofore made on the
Bonds of Series S shall be duly noted thereon by the Trustee.

    It is expected that the Company, pursuant to the Pennington Agreement 
No. 1, will furnish directly to the County Trustee at its principal corporate
trust office all funds required for any and all payments of principal of, and
interest and premium on, the Pennington Series A Bonds (or that in lieu of any
such payment the Company or Pennington will deposit with the County Trustee
Pennington Series A Bonds for cancellation) and that corresponding payments of
interest and of installments of principal (including premiums if appropriate)
on the Bonds of Series S will automatically be effected in accordance with the
provisions of the Pennington Agreement No. 1. Whenever payment or provisions
therefor has been made in respect of the principal, premium, if any, or
interest on all or any portion of the Pennington Series A Bonds in accordance
with the Pennington Pollution Control Indenture, the corresponding principal
installment, premium, if any, or interest on the Bonds of Series S shall be
deemed paid to the extent such payment or provision therefor has been made.
Unless payment then is or has been made as above provided, payment of the
principal of, and
<PAGE>
 
                                      19
 







 
 
premium, if any, and interest on the single Bond of Series S shall be made to
the County Trustee for the account of Pennington in funds immediately 
available at said office of the County Trustee, in each case on or prior to the
due date for such payment. The Trustee may at any and all times conclusively 
assume that the obligation of the Company to make payments with respect to the 
principal of and premium, if any, and interest on the Bonds of Series S, so far
as such payments at the time have become due, has been fully satisfied and 
discharged unless and until the Trustee shall have received a written notice
from the County Trustee signed by one of its officers stating (i) that timely
payment of principal of, or premium or interest on, Bonds of Series S has not
been made, (ii) that the Company is in arrears as to the payments required to
be made by it to the County Trustee pursuant to the Pennington Agreement No. 1
after giving effect to any available moneys in the Bond Fund provided by the
Pennington Pollution Control Indenture and (iii) the amount of the arrearage.

    The County Trustee, by acceptance of the single Bond of Series S, shall 
agree to make prompt notation thereon of all payments and prepayments on 
account of principal thereof made or occurring under any provisions of the 
Pennington Agreement No. 1 or of this Supplemental Indenture, and to 
surrender said Bond to the Trustee upon final payment thereof. If and to the 
extent payment of the Pennington Series A Bonds is deemed to have been made 
pursuant to the terms of the Pennington Pollution Control Indenture, payment of 
the Bonds of Series S shall be deemed to have been made to the same extent.

    The Trustee is hereby appointed Registrar in respect of the Bonds of 
Series S, and the principal corporate trust office of the Trustee in the
Borough of Manhattan, The City of New York, is hereby designated as the office
or agency of the Company in said Borough where notices or demands in respect
of Bonds of Series S may be served.

    SECTION 2.03. The text of the Bonds of Series S, and the certificate 
of authentication of the Trustee to be executed thereon, are to be
substantially in the following forms, respectively: 
<PAGE>
 
                                      20
 






 No. S ........                                                     $2,050,000

                     BLACK HILLS POWER AND LIGHT COMPANY

           FIRST MORTGAGE BOND SERIES S, 6-5/8%, DUE APRIL 1, 2007


    BLACK HILLS POWER AND LIGHT COMPANY (hereinafter called the "Company"), a 
corporation organized and existing under the laws of the State of South Dakota,
for value received, hereby promises to pay to NORTHWESTERN NATIONAL BANK OF 
MINNEAPOLIS as trustee or its successor in trust (the "County Trustee"),
under an Indenture of Trust No. 1 dated as of April 1, 1977 (the "Pennington
Pollution Control Indenture") between Pennington County, South Dakota and
said County Trustee, TWO MILLION FIFTY THOUSAND DOLLARS,($2,050,000) payable in
installments as follows:

<TABLE>
<CAPTION>

     April 1            Principal         April 1              Principal
   Of the Year           Amount         Of the Year              Amount
   -----------          ---------       -----------            ----------
<S>                     <C>             <C>                    <C>
      1993............ $190,000.00         2001.............. $190,000.00
      1994............. 190,000.00         2002..............  190,000.00
      1995............. 190,000.00         2003..............   30,000.00
      1996............. 190,000.00         2004..............   30,000.00
      1997............. 190,000.00         2005..............   30,000.00
      1998............. 190,000.00         2006..............   30,000.00
      1999............. 190,000.00         2007..............   30,000.00
      2000............. 190,000.00
</TABLE>
 
 
together with interest thereon from April 1, 1977 on the unpaid principal
amount of this Bond at the rate of 6 5/8% per annum until due and payable,
such interest to be paid semi-annually in arrears on October 1 and April 1 in
each year. Overdue principal and overdue installments of interest shall bear
interest at the rate of 6% per annum
<PAGE>
 
                                      21
 







(to the extent that payment of such interest is enforceable under applicable
law).

    This Bond is one of an authorized issue of Bonds of the Company known as 
its "First Mortgage Bonds", issued and to be issued in one or more series
under, and all equally and ratably secured (except as any sinking,
amortization, improvement, renewal or other analogous fund, established in
accordance with the provisions of the Indenture hereinafter mentioned, may
afford additional security for the Bonds of any particular series) by, an
Indenture of Mortgage and Deed of Trust dated as of September 1, 1941 as
supplemented and amended by Supplemental Indentures dated as of July l5, 1945,
January 15, 1948, January 15, 1949, March 1, 1950, March 1, 1952, July 1,
1956, May 1, 1957, May 1, 1959, April 1, 1960, August 1, 1960, June 1, 1961,
October 1, 1962, May 1, 1963, June 1, 1969, June 15, 1974, August 1, 1974,
July 15, 1975, May 1, 1976 and February 15, 1977 and subordination agreements
dated December 29, 1950 and September 19, 1955 (hereinafter collectively
called the "Indenture"), executed by the Company to Manufacturers Hanover
Trust Company (herein called the "Trustee"), as Trustee, to which Indenture
and all indentures supplemental thereto reference is hereby made for a
description of the properties mortgaged and pledged, the nature and extent of
the security, the rights of the holders of said Bonds and the coupons
appurtenant to coupon Bonds and of the Trustee and of the Company in respect
of such security, and the terms and conditions upon which said Bonds are and
are to be issued and secured.


     To the extent permitted by the Indenture and as provided therein, with the 
consent of the Company and upon the written consent or affirmative vote of at 
least sixty-six and two-thirds per cent. in principal amount of the Bonds then 
outstanding and entitled to consent, and of not less than sixty-six and
two-thirds per cent. in principal amount of the Bonds then outstanding and
entitled to consent of each series affected thereby in case one or more but
less than all of the series of Bonds issued under the Indenture are so
affected, the rights and obligations of the Company and of the holders of
Bonds and coupons appurtenant to coupon Bonds, and the terms and provisions of 
<PAGE>
 
                                      22








 
 
the Indenture and of any instrument supplemental thereto may be modified from 
time to time, provided that no such modification or alteration shall be made
which would postpone the date fixed herein or in the coupons or in the
Indenture for the payment of the principal of, or any installment of interest
on, the Bonds, or reduce the principal of, or the rate of interest payable on,
the Bonds, or reduce the percentage of the principal amount of Bonds the
consent of which is required for the authorization of any such modification or
alteration, or which would modify, without the written consent of the Trustee,
the rights, duties or immunities of the Trustee.

    As provided in said Indenture, said Bonds are issuable in series which may
vary as in said Indenture provided or permitted. This Bond is one of a series of
Bonds entitled "First Mortgage Bonds Series S".

    Pennington County, South Dakota (the "County") is issuing and selling, or
has issued and sold, its Pollution Control Revenue Bonds (Black Hills Power and
Light Company Project) Collateralized 1977 Series A (hereinafter called the
"Pennington Series A Bonds") under and pursuant to the Pennington Pollution
Control Indenture. This Bond of Series S is issued as a means of securing
payment of the purchase price of the Company's purchase from the County of
certain pollution control facilities pursuant to an Agreement of Sale No. 1
dated as of April 1, 1977 (the "Pennington Agreement No. 1"). It is expected
that the Company will make all payments of principal of, and interest and
premium on, the Pennington Series A Bonds directly to the County Trustee (or
that in lieu thereof the Company or Pennington will deposit with the County
Trustee Pennington Series A Bonds for cancellation). All such payments shall
automatically constitute corresponding payments on the Bonds of Series S in
accordance with the provisions of the Pennington Agreement No. 1. The holder of
this Bond by acceptance hereof agrees that whenever any payment or prepayment on
account of the principal of this Bond is made or occurs under any provision of
the Indenture, the Pennington Agreement No. 1 or the Pennington Pollution
Control Indenture, the holder hereof shall promptly note on the Schedule of
Payments of Principal, the Schedule of Prepayments of Principal or the Schedule
of Credits of Prepayments of Principal the date and amount of each such payment
or pre-
<PAGE>
 
                                      23
 



 
payment of principal, and shall promptly notify the Trustee of the amount of
each such payment and that the notation of payment has been duly made, and
further agrees to surrender this Bond to the Trustee for cancellation when
all principal of, premium, if any, and interest on this Bond shall have been
duly paid. The holder of this Bond further agrees that if payment of the
Pennington Series A Bonds is deemed to have been made pursuant to the
provisions of the Pennington Pollution Control Indenture, payment of this Bond
shall be deemed to have been made.
 
    Unless payment then is or has been made pursuant to the foregoing paragraph,
the principal of and premium, if any, and interest on this Bond will be paid in
lawful money of the United States of America and will be payable at the
principal corporate trust office of the County Trustee to the County Trustee for
the account of Pennington in immediately available funds at said office of the
County Trustee, in each case on or prior to the due date for such payment. The
holder of this Bond by acceptance hereof agrees that any such payment of
principal, premium or interest on this Bond shall be credited as and used to
make a corresponding payment of principal, premium or interest on the Pennington
Series A Bonds.
 
    This Bond is not prepayable prior to April 1, 1987, except that if
at any time (1) the Company shall elect to prepay installments payable under
the Pennington Agreement No. 1 and to cause the Pennington Series A Bonds to be
redeemed upon the occurrence of an event specified in Section 9.1 of the
Pennington Agreement No. 1 or (2) the Company shall be obligated to prepay
installments payable under the Pennington Agreement No. 1 and to cause the
Pennington Series A Bonds to be redeemed upon the occurrence of an event
specified in Section 9.2 of the Pennington Agreement No. 1, in any such event
this Bond shall be prepaid by the Company in whole, but not in part (except
that upon the occurrence of an event specified in paragraph (c) of Section 9.2
of the Pennington Agreement No. 1 the Bond shall be prepaid in whole or in
part) at 100% of the unpaid principal amount thereof plus accrued interest to
the prepayment date, which date shall be the same date as the prepayment date
under the Pennington Agreement No. 1 determined as set forth in said Sections
9.1 and 9.2. The principal amount of Bonds of Series S to be prepaid at any
time upon
<PAGE>
 
                                      24

 
 
 
the occurrence of the event specified in paragraph (c) of Section 9.2 of the 
Pennington Agreement No. 1 shall be the same as the principal amount of 
Pennington Series A Bonds to be redeemed as the result of the occurrence of 
such event.
 
    This Bond shall be prepaid by the Company on or after April 1, 1987, in 
whole at any time or in part on any interest payment date, if but only if the 
Company shall have elected to prepay installments under the Pennington 
Agreement No. 1 of like principal amount and to cause Pennington Series A 
Bonds of like principal amount to be redeemed on the prepayment date in 
accordance with the second paragraph of Section 3.01 of the Pennington 
Pollution Control Indenture. Such prepayment shall be at the prepayment price 
determined in accordance with the following table plus accrued interest to the 
prepayment date:



<TABLE>
<CAPTION>
If prepayment date is during                              
    twelve-month period                                    Prepayment
     beginning April 1                                        Price
- ----------------------------                               ----------
<S>                                                        <C>

           1987.............................................. 103    %
           1988.............................................. 102 1/2
           1989.............................................. 102
           1990.............................................. 101 1/2
           1991.............................................. 101
           1992.............................................. 100 1/2
           1993 and thereafter ...............................100
</TABLE>
 
    Except to the extent that the Company shall at any time elect, pursuant to
the provisions of the Pennington Pollution Control Indenture, to apply as a
credit in respect of a sinking fund obligation thereunder an amount of
Pennington Series A Bonds redeemed or purchased and delivered to the County
Trustee, any prepayment of only a part of this Bond shall be in inverse order of
the maturities of the several installment payments, e.g., all the portion of
this Bond due on April 1, 2007, must be prepaid before any installment having an
earlier maturity date. In the event that the Company at any time elects,
pursuant to the provisions of the Pennington Pollution Control Indenture, to
apply as a credit in respect of a sinking fund obligation thereunder an amount
of Pennington Series A Bonds redeemed or purchased and delivered to the County
Trustee, the corresponding prepayment of this Bond in the same principal amount
shall be credited against the installment due on this Bond in the same year as
such sinking fund obligation.
<PAGE>
 
                                      25
 







    In each case where this Bond is to be prepaid in whole or in part as 
contemplated herein notice of not less than thirty (30) nor more than sixty
(60) days shall be given by first class mail postage prepaid to the holder of
record of this Bond unless such notice has been waived in writing by the
County Trustee. 


    If an event of default as defined in the Indenture shall occur, the
principal of this Bond may become and be declared due and payable in the manner
and with the effect provided in the Indenture. No holder of this Bond shall
have any right to institute any suit or proceeding for the foreclosure of the
Indenture or for any other remedy thereunder, except to the extent and in the
manner set forth in the Indenture.

    This Bond shall be non-transferable except as required to effect (i) a 
transfer to any successor trustee under the Pennington Pollution control 
Indenture or (ii) a transfer after an event of default under the Pennington 
Pollution Control Indenture in the course of the exercise of rights and
remedies consequent upon such event of default. Subject to such restrictions,
this Bond is transferable by the registered holder hereof in person or by
attorney duly authorized in writing, at the office of Manufacturers Hanover
Trust Company, New York, New York, on registry books kept for such purpose at
such office. No charge will be made by the Company for any such transfer of
this Bond. Before any transfer of this Bond by the County Trustee or attorney
duly authorized will be recognized or given effect by the Company or the
Trustee, the County Trustee shall note hereon the date to which interest has
been paid as well as the amounts of all principal payments and prepayments
hereon, and shall notify the Company and the Trustee of the name and address
of the transferee and shall afford the Company and the Trustee the opportunity
of verifying the notation as to payment of interest and principal.

    No recourse shall be had for the payment of the principal of or the
interest on this Bond or for any claim based hereon or otherwise in respect
hereof or of the Indenture or of any agreement supplemental thereto against
any subscriber to the capital stock, incorporator or any past, present or
future stockholder, officer or director of the Company or of any predecessor
or successor corporation, either directly or through the Company or any
predecessor or successor corporation or any receiver or trustee in bankruptcy,
whether by virtue of any constitution or statute or rule of law or by the
enforcement of any assess-
<PAGE>
 
                                      26
 



ment or penalty or stock subscription or otherwise, all such liability, whether
at common law or in equity or by statute or constitution or otherwise, being,
to the extent permitted by law, by the acceptance and as a part of the
consideration for the issuance hereof, expressly waived and released by the
registered holder hereof.

    This Bond shall not be valid or become obligatory for any purpose until the 
certificate of authentication hereon shall have been signed by the Trustee, or
its successor as Trustee, under said Indenture.

    IN WITNESS WHEREOF, the Company has caused this Bond to be signed in 
its name by its President or one of its Vice-Presidents, and its corporate seal
to be impressed or imprinted hereon and attested by its Secretary or one of
its Assistant Secretaries. 


    Dated,
                                         BLACK HILLS POWER AND LIGHT COMPANY, 
 
                                         BY.................................. 
Attest:                                                            President.

......................................
                            Secretary.
 
                       (FORM OF TRUSTEE'S CERTIFICATE)
 
    This is one of the Bonds, of the series designated therein, described in
the within mentioned Indenture.

                                         MANUFACTURERS HANOVER TRUST COMPANY, 
                                                                 as Trustee, 
 
                                         BY .................................. 
                                                           Authorized Officer. 
<PAGE>
 
                                      27
 







 
                      SCHEDULE OF PAYMENTS OF PRINCIPAL
<TABLE>
<CAPTION>

 Amount of              Date              Date of            Authorized Official
Installment             Due               Payment                 and Title 
- -----------             ----              -------            -------------------
<S>                     <C>               <C>                <C> 
</TABLE> 


 
                     SCHEDULE OF PREPAYMENTS OF PRINCIPAL


<TABLE>
<CAPTION>
Principal Amount                                             Authorized Official
    Prepaid                     Date Prepaid                      and Title
- ----------------                ------------                 -------------------
<S>                             <C>                          <C>
</TABLE>
 
 
                            SCHEDULE OF CREDITS OF
                          PREPAYMENTS OF PRINCIPAL*


<TABLE>
<CAPTION>

                    Date of Installment 
Prepaid Principal        to which                            Authorized Official
Amount Credited          Credited         Date Credited           and Title
- -----------------   -------------------   -------------      -------------------
<S>                 <C>                   <C>                <C>
</TABLE>


_________
    *This Schedule to be completed only in the event that a prepayment is
credited against an installment maturing prior to the then last maturing
unpaid installment. Each prepayment is credited against the then last maturing
unpaid installment unless Pennington Series A Bonds redeemed or purchased and
delivered to the County Trustee are credited against a sinking fund obligation
under the Pennington Pollution Control Indenture, whereupon a corresponding 
prepayment in the same principal amount shall be credited against the
installment due in the same year as such sinking fund obligation and entered
in this Schedule. 


    (NOTICE: The within Bond may not be transferred until this schedule has
been verified by the Trustee.) 
<PAGE>
 
                                      28





    SECTION 2.04. The single Bond of Series S is not prepayable prior to April
1,  1987, except that if, at any time, (1) the Company shall elect to prepay 
installments payable under the Pennington Agreement No. 1 and to cause the 
Pennington Series A Bonds to be redeemed upon the occurrence of an event 
specified in Section 9.1 of the Pennington Agreement No. 1 or (2) the Company 
shall be obligated to prepay installments payable under the Pennington 
Agreement No. 1 and to cause the Pennington Series A Bonds to be redeemed 
upon the occurrence of an event specified in Section 9.2 of the Pennington 
Agreement No. 1, in any such event the Bonds of Series S shall be prepaid by 
the Company in whole, but not in part (except that upon the occurrence of an 
event specified in paragraph (c) of Section 9.2 of the Pennington Agreement 
No. 1 the Bonds of Series S shall be prepaid in whole or in part) at 100% of
the unpaid principal amount thereof plus accrued interest to the prepayment
date, which date shall be the same date as the prepayment date under the
Pennington Agreement No. 1 determined as set forth in said Sections 9.1 and
9.2. The principal amount of Bonds of Series S to be prepaid at any time upon
the occurrence of the event specified in paragraph (c) of Section 9.2 of the 
Pennington Agreement No. 1 shall be the same as the principal amount of 
Pennington Series A Bonds to be redeemed as the result of the occurrence of 
such event.

    The single Bond of Series S shall be prepaid by the Company on or after 
April 1, 1987, in whole at any time or in part on any interest payment date, if 
but only if the company shall have elected to prepay installments under the 
Pennington Agreement No. 1 of like principal amount and to cause Pennington 
Series A Bonds of like principal amount to be redeemed on the prepayment 
date in accordance with the second paragraph of Section 3.01 of the 
Pennington Pollution Control Indenture. Such prepayment shall be at the 
prepayment price determined in accordance with the following table plus 
accrued interest to the prepayment date:


<TABLE>
<CAPTION>

     If the prepayment date is
     during the twelve months                            Prepayment
        beginning April 1                                  Price
     -------------------------                           -----------
<S>                                                      <C>
 
              1987......................................... 103    %
              1988......................................... 102 1/2
              1989......................................... 102
              1990......................................... 101 1/2
              1991......................................... 101
              1992......................................... 100 1/2
              1993 and thereafter ......................... 100
</TABLE>
<PAGE>
 
                                      29
 



    Except to the extent that the Company shall at any time elect, pursuant to 
the provisions of the Pennington Pollution Control Indenture, to apply as a
credit in respect of a sinking fund obligation thereunder an amount of
Pennington Series A Bonds redeemed or purchased and delivered to the County
Trustee, any prepayment of only a part of the single Bond of Series S shall be
in inverse order of the maturities of the several installment payments, e.g.,
all the portion of the single Bond of Series S due on April 1, 2007, must be
prepaid before any installment having an earlier maturity date. In the event
that the Company at any time elects, pursuant to the provisions of the
Pennington Pollution Control Indenture, to apply as a credit in respect of a
sinking fund obligation thereunder an amount of Pennington Series A Bonds
redeemed or purchased and delivered to the County Trustee, the corresponding
prepayment of the single Bond of Series S in the same principal amount shall
be credited against the installment due on the single Bond of Series S in the
same year as such sinking fund obligation.


    If Bonds of Series S are to be redeemed in whole or in part as provided in 
this Section 2.04, notice of redemption shall be given by first class mail,
postage prepaid, by or on behalf of the Company, not less than thirty (30) nor
more than sixty (60) days prior to the date of redemption, to the registered
holders of all Bonds so to be redeemed, at their respective addresses
appearing upon the books maintained by the Trustee pursuant to Section 2.10 of
the Original Indenture. Any notice which is mailed as herein provided shall be
conclusively presumed to have been properly and sufficiently given on the date
of such mailing, whether or not the registered holder or payee, as the case
may be, receives the notice. In case of any redemption of Bonds of such Series
by the Trustee pursuant to the provisions of the Indenture or any indenture
supplemental thereto, notice of redemption shall be given in a similar manner
by the Trustee. Notwithstanding any provision of Article Ten of the Original
Indenture, no publication of notice of redemption of any Bonds of Series S
shall be required by the Indenture. 


    Except as provided in the immediately preceding paragraph, the provisions 
of Article Ten of the Original Indenture shall in all respects apply to any 
redemption to which this Section 2.04 applies. Nothing in this Section 2.04 
contained shall affect the manner of giving notice of the redemption of Bonds
of  the Company of any series other than Bonds of Series S. 
<PAGE>
 
                                      30
 







 
    Notwithstanding the provisions of the Original Indenture (including 
particularly Section 8.08 (b) thereof), the single Bond of Series S is subject
to redemption prior to maturity only upon the events and as set forth in this
Section 2.04. 

                                ARTICLE THREE
 
                           BONDS OF SERIES T, 6 5/8%
 
 
    SECTION 3.01. There is hereby created a series of Bonds, known as and
entitled "First Mortgage Bonds Series T, 6 5/8%", due April 1, 2007 and the form
thereof shall be as provided in this Supplemental Indenture.

    SECTION 3.02. The Bonds of Series T shall be evidenced by a single 
registered bond in the principal amount and denomination of One Million Dollars 
($1,000,000) finally due April 1, 2007 but payable in installments as follows: 

<TABLE>
<CAPTION>

     April 1            Principal         April 1              Principal
   Of the Year           Amount         Of the Year              Amount
   -----------          ---------       -----------            ---------
<S>                    <C>              <C>                    <C>
      1993............ $80,000.00           2001..............  $80,000.00
      1994............  80,000.00           2002..............   80,000.00
      1995............  80,000.00           2003..............   40,000.00
      1996............  80,000.00           2004..............   40,000.00
      1997............  80,000.00           2005..............   40,000.00
      1998............  80,000.00           2006..............   40,000.00
      1999............  80,000.00           2007..............   40,000.00
      2000............  80,000.00
</TABLE>


The Bonds of Series T shall bear interest on the unpaid principal balance at
the rate of 6 5/8% per annum payable semi-annually on October 1 and April 1 
commencing October 1, 1977 until due and payable and shall bear interest at 
the rate of 6% per annum on any overdue principal and on any overdue 
installment of interest (to the extent that payment of such interest is
enforceable under applicable law). April 1, 1977 shall be the date of
commencement of the first interest period for such Bonds. All such bonds shall
be dated as provided in Section 2.05 of the Original Indenture.
<PAGE>
 
                                      31
 






 
    The single Bond of Series T shall be lettered T and numbered 1 and shall, 
upon issuance, be delivered by the Company to and registered in the name of the 
County Trustee and shall be transferable only as required to effect an 
assignment thereof to a successor County Trustee. Bonds issued upon transfer 
shall be lettered T and numbered from 2 upwards and issued in the same 
$1,000,000 denomination but all payments of principal theretofore made on the 
Bonds of Series T shall be duly noted thereon by the Trustee.

    It is expected that the Company pursuant to the Lawrence Agreement No. 
2 will furnish directly to the County Trustee at its principal corporate trust
office all funds required for any and all payments of principal of, and
interest and premium on, the Lawrence Series B Bonds (or that in lieu of any
such payment the Company or Lawrence will deposit with the County Trustee 
Lawrence Series B Bonds for cancellation) and that corresponding payments of 
interest and of installments of principal (including premiums if appropriate)
on the Bonds of Series T will automatically be effected in accordance with the 
provisions of the Lawrence Agreement No. 2. Whenever payment or provision 
therefor has been made in respect of the principal, premium, if any, or
interest on all or any portion of the Lawrence Series B Bonds in accordance
with the Lawrence Revenue Bond Indenture, the corresponding principal
installment, premium, if any, or interest on the Bonds of Series T shall be
deemed paid to the extent such payment or provision therefor has been made.
Unless payment then is or has been made as above provided, payment of the
principal of, and premium, if any, and interest on the single Bond of Series T
shall be made to the County Trustee for the account of Lawrence in funds
immediately available at said office of the County Trustee, in each case on or
prior to the due date for such payment. The Trustee may at any and all times
conclusively assume that the obligation of the Company to make payments with
respect to the principal of and premium, if any, and interest on the Bonds of
Series T, so far as such payments at the time have become due, has been fully
satisfied and discharged unless and until the Trustee shall have received a
written notice from the County Trustee signed by one of its officers stating
(i) that timely payment of principal of, or premium or interest on, Bonds of
Series T has not been made, (ii) that the Company is in arrears as to the
payments required to be made by it to the County Trustee pursuant to the
Lawrence
<PAGE>
 
                                      32
 





Agreement No. 2 after giving effect to any available moneys in the Bond Fund 
provided by the Lawrence Revenue Bond Indenture and (iii) the amount of the 
arrearage.

    The County Trustee, by acceptance of the single Bond of Series T, shall 
agree to make prompt notation thereon of all payments and prepayments on 
account of principal thereof made or occurring under any provisions of the 
Lawrence Agreement No. 2 or of this Supplemental Indenture, and to surrender 
said Bond to the Trustee upon final payment thereof. If and to the extent 
payment of the Lawrence Series B Bonds is deemed to have been made pursuant to
the terms of the Lawrence Revenue Bond Indenture, payment of the Bonds of
Series T shall be deemed to have been made to the same extent.

    The Trustee is hereby appointed Registrar in respect of the Bonds of 
Series T, and the principal corporate trust office of the Trustee in the
Borough of Manhattan, The City of New York, is hereby designated as the office
or agency of the Company in said Borough where notices or demands in respect
of Bonds of Series T may be served.

    SECTION 3.03. The text of the Bonds of Series T, and the certificate of 
authentication of the Trustee to be executed thereon, are to be substantially
in  the following forms, respectively. 


 No. T..........                                                    $1,000,000

                     BLACK HILLS POWER AND LIGHT COMPANY

            FIRST MORTGAGE BOND SERIES T, 6 5/8%, DUE APRIL 1, 2007


    BLACK HILLS POWER AND LIGHT COMPANY (hereinafter called the 
"Company"), a corporation organized and existing under the laws of the State
of South Dakota, for value received, hereby promises to pay to NORTHWESTERN
NATIONAL BANK OF MINNEAPOLIS as trustee or its successor in trust (the
"County Trustee"), under an Indenture of Trust No. 2 dated as of April 1,
1977 (the "Lawrence Revenue Bond Indenture") between Lawrence County, South
Dakota and said County Trustee, ONE MILLION DOLLARS, ($1,000,000) payable in
installments as follows: 
<PAGE>
 
                                      33







<TABLE>
<CAPTION>

     April 1            Principal         April 1              Principal
   Of the Year           Amount         Of the Year              Amount
   -----------          ---------       -----------            ---------
<S>                    <C>              <C>                    <C>
      1993............ $80,000.00           2001..............  $80,000.00
      1994............  80,000.00           2002..............   80,000.00
      1995............  80,000.00           2003..............   40,000.00
      1996............  80,000.00           2004..............   40,000.00
      1997............  80,000.00           2005..............   40,000.00
      1998............  80,000.00           2006..............   40,000.00
      1999............  80,000.00           2007..............   40,000.00
      2000............  80,000.00
</TABLE>
 
together with interest thereon from April 1, 1977 on the unpaid principal
amount of this Bond at the rate of 6 5/8% per annum until due and payable, such
interest to be paid semi-annually in arrears on October 1 and April 1 in each
year. Overdue principal and overdue installments of interest shall bear
interest at the rate of 6% per annum (to the extent that payment of such
interest is enforceable under applicable law).
 
    This Bond is one of an authorized issue of Bonds of the Company known 
as its "First Mortgage Bonds", issued and to be issued in one or more series 
under, and all equally and ratably secured (except as any sinking,
amortization, improvement, renewal or other analogous fund, established in
accordance with the provisions of the Indenture hereinafter mentioned, may
afford additional security for the Bonds of any particular series) by, an
Indenture of Mortgage and Deed of Trust dated as of September 1, 1941 as
supplemented and amended by Supplemental Indentures dated as of July 15, 1945,
January 15, 1948, January 15, 1949, March 1, 1950, March 1, 1952, July 1,
1956, May 1, 1957, May 1, 1959, April 1, 1960, August 1, 1960, June 1, 1961,
October l, 1962, May l, 1963, June 1, 1969, June 15, 1974, August 1, 1974,
July 15, 1975, May 1, 1976 and February 15, 1977 and subordination agreements
dated December 29, 1950 and September 19, 1955 (hereinafter collectively
called the "Indenture"), executed by the Company to Manufacturers Hanover
Trust Company (herein called the "Trustee"), as Trustee, to which Indenture
and all indentures supplemental thereto reference is hereby made for a
description of the properties mortgaged and pledged, the nature and extent of
the security, the rights of the holders of said Bonds and the coupons
appurtenant to coupon Bonds 
<PAGE>
 
                                      34
 


and of the Trustee and of the Company in respect of such security, and the 
terms and conditions upon which said Bonds are and are to be issued and 
secured.

    To the extent permitted by the Indenture and as provided therein, with the 
consent of the Company and upon the written consent or affirmative vote of at 
least sixty-six and two-thirds per cent. in principal amount of the Bonds then 
outstanding and entitled to consent, and of not less than sixty-six and
two-thirds per cent. in principal amount of the Bonds then outstanding and
entitled to consent of each series affected thereby in case one or more but
less than all of the series of Bonds issued under the Indenture are so
affected, the rights and obligations of the Company and of the holders of
Bonds and coupons appurtenant to coupon Bonds, and the terms and provisions of
the Indenture and of any instrument supplemental thereto may be modified from
time to time, provided that no such modification or alteration shall be made
which would postpone the date fixed herein or in the coupons or in the
Indenture for the payment of the principal of, or any installment of interest
on, the Bonds, or reduce the principal of, or the rate of interest payable on,
the Bonds, or reduce the percentage of the principal amount of Bonds the
consent of which is required for the authorization of any such modification or
alteration, or which would modify, without the written consent of the Trustee,
the rights, duties or immunities of the Trustee.


    As provided in said Indenture, said Bonds are issuable in series which may 
vary as in said Indenture provided or permitted. This Bond is one of a series
of Bonds entitled "First Mortgage Bonds Series T".

    Lawrence County, South Dakota (the "County") is issuing and selling, or
has issued and sold, its Industrial Development Revenue Bonds (Black Hills
Power and Light Company Project) Collateralized 1977 Series B (hereinafter
called the "Lawrence Series B Bonds") under and pursuant to the Lawrence
Revenue Bond Indenture. This Bond of Series T is issued as a means of securing
payment of the purchase price of the Company's purchase from the County of
certain facilities pursuant to an Agreement of Sale No. 2 dated as of April 1,
1977 (the "Lawrence Agreement No. 2"). It is expected that the Company will
make all payments of principal of, and interest and premium on, the Lawrence
Series B Bonds directly to the County Trustee (or that
<PAGE>
 
                                      35
 





in lieu thereof the Company or Lawrence will deposit with the County Trustee 
Lawrence Series B Bonds for cancellation). All such payments shall 
automatically constitute corresponding payments on the Bonds of Series T in 
accordance with the provisions of the Lawrence Agreement No. 2. The holder of
this Bond by acceptance hereof agrees that whenever any payment or prepayment
on account of the principal of this Bond is made or occurs under any provision
of the Indenture, the Lawrence Agreement No. 2 or the Lawrence Revenue Bond
Indenture, the holder hereof shall promptly note on the Schedule of Payments
of Principal, the Schedule of Prepayments of Principal or the Schedule of
Credits of Prepayments of Principal the date and amount of each such payment
or prepayment of principal, and shall promptly notify the Trustee of the
amount of each such payment and that the notation of payment has been duly
made, and further agrees to surrender this Bond to the Trustee for 
cancellation when all principal of, premium, if any, and interest on this Bond
shall have been duly paid. The holder of this Bond further agrees that if
payment of the Lawrence Series B Bonds is deemed to have been made pursuant to
the provisions of the Lawrence Revenue Bond Indenture, payment of this Bond
shall be deemed to have been made.

    Unless payment then is or has been made pursuant to the foregoing 
paragraph, the principal of and premium, if any, and interest on this Bond will
be paid in lawful money of the United States of America and will be payable at
the principal corporate trust office of the County Trustee to the County
Trustee for the account of Lawrence in immediately available funds at said
office of the County Trustee, in each case on or prior to the due date for
such payment. The holder of this Bond by acceptance hereof agrees that any
such payment of principal, premium or interest on this Bond shall be credited
as and used to make a corresponding payment of principal, premium or interest
on the Lawrence Series B Bonds.

    This Bond is not prepayable prior to April 1, 1987, except that if at any
time (1) the Company shall elect to prepay installments payable under the
Lawrence Agreement No. 2 and to cause the Lawrence Series B Bonds to be 
redeemed upon the occurrence of an event specified in Section 9.1 of the 
Lawrence Agreement No. 2 or (2) the Company shall be obligated to prepay 
installments payable under the Lawrence Agreement No. 2 and to cause the 
Lawrence Series B Bonds to be
<PAGE>
 
                                      36
 






redeemed upon the occurrence of an event specified in Section 9.2 of the 
Lawrence Agreement No. 2, in any such event this Bond shall be prepaid by the 
Company in whole, but not in part (except that upon the occurrence of an event 
specified in paragraph (c) of Section 9.2 of the Lawrence Agreement No. 2 the 
Bond shall be prepaid in whole or in part) at of the unpaid principal amount 
thereof plus accrued interest to the prepayment date, which date shall be the 
same date as the prepayment date under the Lawrence Agreement No. 2 determined
as set forth in said Sections 9.1 and 9.2. The principal amount of Bonds of
Series T to be prepaid at any time upon the occurrence of the event specified
in paragraph (c) of Section 9.2 of the Lawrence Agreement No. 2 shall be the
same as the principal amount of Lawrence Series B Bonds to be redeemed as the
result of the occurrence of such event.

 This Bond shall be prepaid by the Company on or after April 1, 1987, in 
whole at any time or in part on any interest payment date, if but only if the 
Company shall have elected to prepay installments under the Lawrence Agreement
No. 2 of like principal amount and to cause Lawrence Series B Bonds of like
principal amount to be redeemed on the prepayment date in accordance with the
second paragraph of Section 3.01 of the Lawrence Revenue Bond Indenture. Such
prepayment shall be at the prepayment price determined in accordance with the
following table plus accrued interest to the prepayment date:

<TABLE>
<CAPTION>
If prepayment date is during 
twelve-month period beginning                   Prepayment
          April 1                                 Price
- -----------------------------                   ----------
<S> <C>
 1987.............................................. 103    %
 1988.............................................. 102 1/2
 1989.............................................. 102
 1990.............................................. 101 1/2
 1991.............................................. 101
 1992.............................................. 100 1/2
 1993 and thereafter ...............................100
</TABLE>
 
 
 Except to the extent that the Company shall at any time elect, pursuant to 
the provisions of the Lawrence Revenue Bond Indenture, to apply as a credit in 
respect of a sinking fund obligation thereunder an amount of Lawrence Series B 
Bonds redeemed or purchased and delivered to the County Trustee, any 
prepayment of only a part of this Bond shall be in inverse order of the
maturities of the several install-
<PAGE>
 
                                      37
 

ment payments, e.g., all the portion of this Bond due on April 1, 2007, must
be prepaid before any installment having an earlier maturity date. In the
event that the Company at any time elects, pursuant to the provisions of the
Lawrence Revenue Bond Indenture, to apply as a credit in respect of a sinking
fund obligation thereunder an amount of Lawrence Series B Bonds redeemed or 
purchased and delivered to the County Trustee, the corresponding prepayment of
this Bond in the same principal amount shall be credited against the
installment due on this Bond in the same year as such sinking fund obligation.

    In each ease where this Bond is to be prepaid in whole or in part as 
contemplated herein notice of not less than thirty (30) nor more than sixty
(60) days shall be given by first class mail postage prepaid to the holder of
record of this Bond unless such notice has been waived in writing by the
County Trustee.

    If an event of default as defined in the Indenture shall occur, the 
principal of this Bond may become and be declared due and payable in the
manner and with the effect provided in the Indenture. No holder of this Bond
shall have any right to institute any suit or proceeding for the foreclosure
of the Indenture or for any other remedy thereunder, except to the extent and
in the manner set forth in the Indenture. 


    This Bond shall be non-transferable except as required to effect (i) 
a transfer to any successor trustee under the Lawrence Revenue Bond Indenture
or (ii) a transfer after an event of default under the Lawrence Revenue Bond
Indenture in the course of the exercise of rights and remedies consequent upon
such event of default. Subject to such restrictions, this Bond is transferable
by the registered holder hereof in person or by attorney duly authorized in
writing, at the office of Manufacturers Hanover Trust Company, New York, New
York, on registry books kept for such purpose at such office. No charge will
be made by the Company for any such transfer of this Bond. Before any
transfer of this Bond by the County Trustee or attorney duly authorized will
be recognized or given effect by the Company or the Trustee, the County
Trustee shall note hereon the date to which interest has been paid as well as
the amounts of all principal payments and prepayments hereon, and shall notify
the Company and the Trustee of the name and address of the transferee and
shall afford the Company and the Trustee the opportunity of verifying the
notation as to payment of interest and principal. 
<PAGE>
 
                                      38





    No recourse shall be had for the payment of the principal of or the
interest on this Bond or for any claim based hereon or otherwise in respect
hereof or of the Indenture or of any agreement supplemental thereto against
any subscriber to the capital stock, incorporator or any past, present or
future stockholder, officer or director of the Company or of any predecessor
or successor corporation, either directly or through the Company or any
predecessor or successor corporation or any receiver or trustee in bankruptcy,
whether by virtue of any constitution or statute or rule of law or by the
enforcement of any assessment or penalty or stock subscription or otherwise,
all such liability, whether at common law or in equity or by statute or
constitution or otherwise, being, to the extent permitted by law, by the
acceptance and as a part of the consideration for the issuance hereof, waived
and released by the registered holder hereof.

    This Bond shall not be valid or become obligatory for any purpose until the 
certificate of authentication hereon shall have been signed by the Trustee, or
its successor as Trustee, under said Indenture.

    IS WITNESS WHEREOF, the Company has caused this Bond to be signed 
in its name by its President or one of its Vice-Presidents, and its corporate 
seal to be impressed or imprinted hereon and attested by its Secretary or one
of its Assistant Secretaries.

    Dated,
                                         BLACK HILLS POWER AND LIGHT COMPANY, 
 
                                         BY.................................. 
Attest:                                                            President.

......................................
                            Secretary.
 
                       (FORM OF TRUSTEE'S CERTIFICATE)
 
    This is one of the Bonds, of the series designated therein, described in
the within mentioned Indenture.

                                         MANUFACTURERS HANOVER TRUST COMPANY, 
                                                                 as Trustee, 
 
                                         BY .................................. 
                                                           Authorized Officer. 
<PAGE>
 
                                      39
 






 
                      SCHEDULE OF PAYMENTS OF PRINCIPAL
<TABLE>
<CAPTION>

 Amount of              Date              Date of          Authorized Official
Installment             Due               Payment               and Title 
- -----------             ----              -------          -------------------
<S>                     <C>               <C>              <C> 
</TABLE> 


 
                     SCHEDULE OF PREPAYMENTS OF PRINCIPAL


<TABLE>
<CAPTION>
Principal Amount                                             Authorized Official
    Prepaid                     Date Prepaid                      and Title
- ----------------                ------------                 ------------------ 
<S>                             <C>                          <C>
</TABLE>
 
 
                            SCHEDULE OF CREDITS OF
                          PREPAYMENTS OF PRINCIPAL*


<TABLE>
<CAPTION>

                    Date of Installment 
Prepaid Principal        to which                            Authorized Official
Amount Credited          Credited         Date Credited           and Title
- -----------------   -------------------   -------------      -------------------
<S>                 <C>                   <C>                <C>
</TABLE>

________
    *This Schedule to be completed only in the event that a prepayment is
credited against an installment maturing prior to the then last maturing
unpaid installment. Each prepayment is credited against the then last maturing
unpaid installment unless Lawrence Series B Bonds redeemed or purchased and
delivered to the County Trustee are credited against a sinking fund obligation
under the Lawrence Revenue Bond Indenture, whereupon a corresponding
prepayment in the same principal amount shall be credited against the
installment due in the same year as such sinking fund obligation and entered
in this Schedule. 


    (NOTICE: The within Bond may not be transferred until this schedule has
been verified  by the Trustee.) 
<PAGE>
 
                                      40
 







 
 
 
    SECTION 3.04. The single Bond of Series T is not prepayable prior to 
April 1, 1987, except that if, at any time, (1) the Company shall elect to
prepay installments payable under the Lawrence Agreement No. 2 and to cause
the Lawrence Series B Bonds to be redeemed upon the occurrence of an event 
specified in Section 9.1 of the Lawrence Agreement No. 2 or (2) the Company 
shall be obligated to prepay installments payable under the Lawrence Agreement 
No. 2 and to cause the Lawrence Series B Bonds to be redeemed upon the 
occurrence of an event specified in Section 9.2 of the Lawrence Agreement No. 
2, in any such event the Bonds of Series T shall be prepaid by the Company in 
whole, but not in part (except that upon the occurrence of an event specified
in paragraph (c) of Section 9.2 of the Lawrence Agreement No. 2 the Bonds of 
Series T shall be prepaid in whole or in part) at 100% of the unpaid principal 
amount thereof plus accrued interest to the prepayment date, which date shall
be the same date as the prepayment date under the Lawrence Agreement No. 2 
determined as set forth in said Sections 9.1 and 9.2. The principal amount of 
Bonds of Series T to be prepaid at any time upon the occurrence of the event 
specified in paragraph (c) of Section 9.2 of the Lawrence Agreement No. 2 shall 
be the same as the principal amount of Lawrence Series B Bonds to be redeemed
as the result of the occurrence of such event. 


    The single Bond of Series T shall be prepaid by the Company on or after 
April 1, 1987, in whole at any time or in part on any interest payment date, if
but only if the Company shall have elected to prepay installments under the 
Lawrence Agreement No. 2 of like principal amount and to cause Lawrence Series
B Bonds of like principal amount to be redeemed on the prepayment date in
accordance with the second paragraph of Section 3.01 of the Lawrence Revenue
Bond Indenture. Such prepayment shall be at the prepayment price determined in
accordance with the following table plus accrued interest to the
prepayment date:


<TABLE>
<CAPTION>

     If the prepayment date is
     during the twelve months                            Prepayment
        beginning April 1                                  Price
     -------------------------                           ---------- 
<S>                                                      <C>
 
              1987......................................... 103    % 
              1988......................................... 102 1/2
              1989......................................... 102
              1990......................................... 101 1/2
              1991......................................... 101
              1992......................................... 100 1/2
              1993 and thereafter ......................... 100
</TABLE>
<PAGE>
 
                                      41
 


    Except to the extent that the Company shall at any time elect, pursuant to 
the provisions of the Lawrence Revenue Bond Indenture, to apply as a credit in 
respect of a sinking fund obligation thereunder an amount of Lawrence Series B 
Bonds redeemed or purchased and delivered to the County Trustee, any 
prepayment of only a part of the single Bond of Series T shall be in inverse
order of the maturities of the several installment payments, e.g., all the
portion of the single Bond of Series T due on April 1, 2007, must be prepaid
before any installment having an earlier maturity date. In the event that the
Company at any time elects, pursuant to the provisions of the Lawrence Revenue
Bond Indenture, to apply as a credit in respect of a sinking fund obligation
thereunder an amount of Lawrence Series B Bonds redeemed or purchased and
delivered to the County Trustee, the corresponding prepayment of the single
Bond of Series T in the same principal amount shall be credited against the
installment due on the single Bond of Series T in the same year as such
sinking fund obligation.


    If Bonds of Series T are to be redeemed in whole or in part as provided in 
this Section 3.04, notice of redemption shall be given by first class mail,
postage prepaid, by or on behalf of the Company, not less than thirty (30) nor
more than sixty (60) days prior to the date of redemption, to the registered
holders of all Bonds so to be redeemed, at their respective addresses
appearing upon the books maintained by the Trustee pursuant to Section 2.10 of
the Original Indenture. Any notice which is mailed as herein provided shall
be conclusively presumed to have been properly and sufficiently given on the
date of such mailing, whether or not the registered holder or payee, as the
case may be, receives the notice. In case of any redemption of Bonds of such
Series by the Trustee pursuant to the provisions of the Indenture or any
indenture supplemental thereto, notice of redemption shall be given in a
similar manner by the Trustee. Notwithstanding any provision of Article Ten of
the Original Indenture, no publication of notice of redemption of any Bond of
Series T shall be required by the Indenture.


    Except as provided in the immediately preceding paragraph, the provisions 
of Article Ten of the Original Indenture shall in all respects apply to any 
redemption to which this Section 3.04 applies. Nothing in this Section 3.04 
contained shall affect the manner of giving notice of the redemption of Bonds 
of the Company of any series other than Bonds of Series T. 
<PAGE>
 
                                      42









    Notwithstanding the provisions of the Original Indenture (including 
particularly Section 8.08(b) thereof), the single Bond of Series T is subject
to redemption prior to maturity only upon the events and as set forth in this
Section 3.04. 

                                 ARTICLE FOUR
 
                          BONDS OF SERIES U, 6 5/8%
 
 
    SECTION; 4.01. There is hereby created a series of Bonds, known as 
and entitled ``First Mortgage Bonds Series U, 6 5/8%'', due April 1, 2007 and
the form thereof shall be as provided in this Supplemental Indenture. 


    SECTION 4.02. The Bonds of Series U shall be evidenced by a single 
registered bond in the principal amount and denomination of One Million Dollars 
($1,000,000) finally due April 1, 2007 but payable in installments as follows: 

<TABLE>
<CAPTION>

     April 1            Principal         April 1              Principal
   Of the Year           Amount         Of the Year              Amount
   -----------          ---------       -----------            ---------
<S>                    <C>              <C>                    <C>
      1993............ $80,000.00           2001..............  $80,000.00
      1994............  80,000.00           2002..............   80,000.00
      1995............  80,000.00           2003..............   40,000.00
      1996............  80,000.00           2004..............   40,000.00
      1997............  80,000.00           2005..............   40,000.00
      1998............  80,000.00           2006..............   40,000.00
      1999............  80,000.00           2007..............   40,000.00
      2000............  80,000.00
</TABLE>

The Bonds of Series U shall bear interest on the unpaid principal balance at
the rate of 6 5/8% per annum payable semi-annually on October 1 and April 1 
commencing October 1, 1977 until due and payable and shall bear interest at 
the rate of 6% per annum on any overdue principal and on any overdue 
installment of interest (to the extent that payment of such interest is
enforceable  under applicable law). April 1, 1977 shall be the date of
commencement of the  first interest period for such Bonds. All such bonds shall
be dated as provided in  Section 2.05 of the Original Indenture. 
<PAGE>
 
                                      43







    The single Bond of Series U shall be lettered U and numbered 1 and shall, 
upon issuance, be delivered by the Company to and registered in the name of the 
County Trustee and shall be transferable only as required to effect an
assignment thereof to a successor County Trustee. Bonds issued upon transfer
shall be lettered U and numbered from 2 upwards and issued in the same
$1,000,000 denomination but all payments of principal theretofore made on the
Bonds of Series U shall be duly noted thereon by the Trustee.

    It is expected that the Company pursuant to the Pennington Agreement 
No. 2 will furnish directly to the County Trustee at its principal corporate
trust office all funds required for any and all payments of principal of, and
interest and premium on, the Pennington Series B Bonds (or that in lieu of any
such payment the Company or Pennington will deposit with the County Trustee 
Pennington Series B Bonds for cancellation) and that corresponding payments of 
interest and of installments of principal (including premiums if appropriate)
on the Bonds of Series U will automatically be effected in accordance with the 
provisions of the Pennington Agreement No. 2. Whenever payment or provision 
therefor has been made in respect of the principal, premium, if any, or
interest on all or any portion of the Pennington Series B Bonds in accordance
with the Pennington Revenue Bond Indenture, the corresponding principal
installment, premium, if any, or interest on the Bonds of Series U shall be
deemed paid to the extent such payment or provision therefor has been made.
Unless payment then is or has been made as above provided, payment of the
principal of, and premium, if any, and interest on the single Bond of Series U
shall be made to the County Trustee for the account of Pennington in funds
immediately available at said office of the County Trustee, in each case on or
prior to the due date for such payment. The Trustee may at any and all times
conclusively assume that the obligation of the Company to make payments with
respect to the principal of and premium, if any, and interest on the Bonds of
Series U, so far as such payments at the time have become due, has been fully
satisfied and discharged unless and until the Trustee shall have received a
written notice from the County Trustee signed by one of its officers stating
(i) that timely payment of principal of, or premium or interest on, Bonds of
Series U has not been made, (ii) that the Company is in arrears as to the
payments required to be made by it to the County 
<PAGE>
 
                                      44
 






Trustee pursuant to the Pennington Agreement No. 2 after giving effect to any 
available moneys in the Bond Fund provided by the Pennington Revenue Bond 
Indenture and (iii) the amount of the arrearage.
 
    The County Trustee, by acceptance of the single Bond of Series U,
shall agree to make prompt notation thereon of all payments and prepayments on
account of principal thereof made or occurring under any provisions of the
Pennington Agreement No. 2 or of this Supplemental Indenture, and to surrender
said Bond to the Trustee upon final payment thereof. If and to the extent
payment of the Pennington Series B Bonds is deemed to have been made pursuant
to the terms of the Pennington Revenue Bond Indenture, payment of the Bonds of
Series U shall be deemed to have been made to the same extent.
 
    The Trustee is hereby appointed Registrar in respect of the Bonds of Series 
U, and the principal corporate trust office of the Trustee in the Borough of 
Manhattan, The City of New York, is hereby designated as the office or agency 
of the Company in said Borough where notices or demands in respect of Bonds of
Series U may be served.
 
    SECTION 4.03. The text of the Bonds of Series U, and the certificate of 
authentication of the Trustee to be executed thereon, are to be substantially
in the following forms, respectively.
 
 No. U.........                                                      $1,000,000
 
                     BLACK HILLS POWER AND LIGHT COMPANY
 
           FIRST MORTGAGE BOND SERIES U, 6 5/8%, DUE  APRIL 1, 2007
 
    BLACK HILLS POWER AND LIGHT COMPANY (hereinafter called the 
"Company"), a corporation organized and existing under the laws of the State
of  South Dakota, for value received, hereby promises to pay to NORTHWESTERN 
NATIONAL BANK OF MINNEAPOLIS as trustee or its successor in trust (the 
"County Trustee"), under an Indenture of Trust No. 2 dated as of April 1,
1977 (the "Pennington Revenue Bond Indenture") between Pennington County,
South Dakota and said County Trustee, ONE MILLION DOLLARS, ($1,000,000) 
payable in installments as follows:
<PAGE>
 
                                      45
 
<TABLE>
<CAPTION>

     April 1            Principal         April 1              Principal
   Of the Year           Amount         Of the Year              Amount
   -----------          ---------       -----------            ---------
<S>                    <C>              <C>                    <C>
      1993............ $80,000.00           2001..............  $80,000.00
      1994............  80,000.00           2002..............   80,000.00
      1995............  80,000.00           2003..............   40,000.00
      1996............  80,000.00           2004..............   40,000.00
      1997............  80,000.00           2005..............   40,000.00
      1998............  80,000.00           2006..............   40,000.00
      1999............  80,000.00           2007..............   40,000.00
      2000............  80,000.00
</TABLE>
 
together with interest thereon from April 1, 1977 on the unpaid principal
amount of this Bond at the rate of 6 5/8% per annum until due and payable, such
interest to be paid semi-annually in arrears on October 1 and April 1 in each
year. Overdue principal and overdue installments of interest shall bear
interest at the rate of 6% per annum (to the extent that payment of such
interest is enforceable under applicable law). 


    This Bond is one of an authorized issue of Bonds of the Company known as
its "First Mortgage Bonds", issued and to be issued in one or more series
under, and all equally and ratably secured (except as any sinking,
amortization, improvement, renewal or other analogous fund, established in
accordance with the provisions of the Indenture hereinafter mentioned, may
afford additional security for the Bonds of any particular series) by, an
Indenture of Mortgage and Deed of Trust dated as of September 1, 1941 as
supplemented and amended by Supplemental Indentures dated as of July 15, 1945,
January 15, 1948, January 15, 1949, March 1, 1950, March 1, 1952, July 1,
1956, May 1, 1957, May 1, 1959, April 1, 1960, August 1, 1960, June 1, 1961, 
October 1, 1962, May 1, 1963, June 1, 1969, June 15, 1974, August 1, 1974,
July 15, 1975, May 1, 1976 and February 15, 1977 and subordination agreements
dated December 29, 1950 and September 19, 1955 (hereinafter collectively called
the "Indenture"), executed by the Company to Manufacturers Hanover Trust
Company (herein called the "Trustee"), as Trustee, to which Indenture and all
indentures supplemental thereto reference is hereby made for a description of
the properties mortgaged and pledged, the nature and extent of the security, the
rights of the holders of said Bonds and the coupons appurtenant to coupon Bonds
<PAGE>
 
                                      46
 






and of the Trustee and of the Company in respect of such security, and the 
terms and conditions upon which said Bonds are and are to be issued and 
secured.

    To the extent permitted by the Indenture and as provided therein, with the 
consent of the Company and upon the written consent or affirmative vote of at 
least sixty-six and two-thirds per cent. in principal amount of the Bonds then 
outstanding and entitled to consent, and of not less than sixty-six and
two-thirds per cent. in principal amount of the Bonds then outstanding and
entitled to consent of each series affected thereby in case one or more but
less than all of the series of Bonds issued under the Indenture are so
affected, the rights and obligations of the Company and of the holders of
Bonds and coupons appurtenant to coupon Bonds, and the terms and provisions of
the Indenture and of any instrument supplemental thereto may be modified from
time to time, provided that no such modification or alteration shall be made
which would postpone the date fixed herein or in the coupons or in the
Indenture for the payment of the principal of, or any installment of interest
on, the Bonds, or reduce the principal of, or the rate of interest payable on,
the Bonds, or reduce the percentage of the principal amount of Bonds the
consent of which is required for the authorization of any such modification or
alteration, or which would modify, without the written consent of the Trustee,
the rights, duties or immunities of the Trustee. 

    As provided in said Indenture, said Bonds are issuable in series which may 
vary as in said Indenture provided or permitted. This Bond is one of a series
of Bonds entitled "First Mortgage Bonds Series U".

    Pennington County, South Dakota (the "County") is issuing and selling,
or has issued and sold, its Industrial Development Revenue Bonds (Black Hills 
Power and Light Company Project) Collateralized 1977 Series B (hereinafter
called the "Pennington Series B Bonds") under and pursuant to the 
Pennington Revenue Bond Indenture. This Bond of Series U is issued as a means
of securing payment of the purchase price of the Company's purchase from the
County of certain facilities pursuant to an Agreement of Sale No. 2 dated as
of April 1, 1977 (the "Pennington Agreement No. 2"). It is expected that the
Company will make all payments of principal of, and interest and premium on, 
the Pennington Series B Bonds directly to the County Trustee (or that in lieu 
thereof the Company or Pennington will deposit 
<PAGE>
 
                                      47




with the County Trustee Pennington Series B Bonds for cancellation). All such 
payments shall automatically constitute corresponding payments on the Bonds of 
Series U in accordance with the provisions of the Pennington Agreement No. 2. 
The holder of this Bond by acceptance hereof agrees that whenever any payment
or prepayment on account of the principal of this Bond is made or occurs under
any provision of the Indenture, the Pennington Agreement No. 2 or the
Pennington Revenue Bond Indenture, the holder hereof shall promptly note on the
Schedule of Payments of Principal, the Schedule of Prepayments of Principal or
the Schedule of Credits of Prepayments of Principal the date and amount of each
such payment or prepayment of principal, and shall promptly notify the Trustee
of the amount of each such payment and that the notation of payment has been
duly made, and further agrees to surrender this Bond to the Trustee for
cancellation when all principal of, premium, if any, and interest on this Bond
shall have been duly paid. The holder of this Bond further agrees that if
payment of the Pennington Series B Bonds is deemed to have been made pursuant
to the provisions of the Pennington Revenue Bond Indenture, payment of this Bond
shall be deemed to have been made.

    Unless payment then is or has been made pursuant to the foregoing 
paragraph, the principal of and premium, if any, and interest on this Bond will
be paid in lawful money of the United States of America and will be payable at
the principal corporate trust office of the County Trustee to the County
Trustee for the account of Pennington in immediately available funds at said
office of the County Trustee, in each case on or prior to the due date for
such payment. The holder of this Bond by acceptance hereof agrees that any
such payment of principal, premium or interest on this Bond shall be credited
as and used to make a corresponding payment of principal, premium or interest
on the Pennington Series B Bonds.

    This Bond is not prepayable prior to April 1, 1987, except that if at any
time (1) the Company shall elect to prepay installments payable under the
Pennington Agreement No. 2 and to cause the Pennington Series B Bonds to be
redeemed upon the occurrence of an event specified in Section 9.1 of the
Pennington Agreement No. 2 or (2) the Company shall he obligated to prepay
installments payable under the Pennington Agreement No. 2 and to cause the
Pennington Series B Bonds to be redeemed upon the occurrence of an event
specified in Sec- 
<PAGE>
 
                                      48







tion 9.2 of the Pennington Agreement No. 2, in any such event this Bond shall be
prepaid by the Company in whole, but not in part (except that upon the
occurrence of an event specified in paragraph (c) of Section 9.2 of the
Pennington Agreement No. 2 the Bond shall be prepaid in whole or in part) at
100% of the unpaid principal amount thereof plus accrued interest to the
prepayment date, which date shall be the same date as the prepayment date under
the Pennington Agreement No. 2 determined as set forth in said Sections 9.1 and
9.2. The principal amount of Bonds of Series U to be prepaid at any time upon
the occurrence of the event specified in paragraph (c) of Section 9.2 of the
Pennington Agreement No. 2 shall be the same as the principal amount of
Pennington Series B Bonds to be redeemed as the result of the occurrence of such
event.

    This Bond shall be prepaid by the Company on or after April 1, 1987, in 
whole at any time or in part on any interest payment date, if but only if the 
Company shall have elected to prepay installments under the Pennington 
Agreement No. 2 of like principal amount and to cause Pennington Series B
Bonds of like principal amount to be redeemed on the prepayment date in 
accordance with the second paragraph of Section 3.01 of the Pennington Revenue
Bond Indenture. Such prepayment shall be at the prepayment price determined in
accordance with the following table plus accrued interest to the prepayment
date:


<TABLE>
<CAPTION>
If prepayment date is during                              
twelve-month-period beginning                              Prepayment
          April 1                                            Price
- -----------------------------                              ----------
<S>                                                        <C>
           1987.............................................. 103    %
           1988.............................................. 102 1/2
           1989.............................................. 102
           1990.............................................. 101 1/2
           1991.............................................. 101
           1992.............................................. 100 1/2
           1993 and thereafter ...............................100
</TABLE>

    Except to the extent that the Company shall at any time elect, pursuant to 
the provisions of the Pennington Revenue Bond Indenture, to apply as a credit
in respect of a sinking fund obligation thereunder an amount of Pennington
Series B Bonds redeemed or purchased 
<PAGE>
 
                                      49






and delivered to the County Trustee, any prepayment of only a part of this Bond 
shall be in inverse order of the maturities of the several installment
payments, e.g., all the portion of this Bond due on April 1, 2007, must be
prepaid before any installment having an earlier maturity date. In the event
that the Company at any time elects, pursuant to the provisions of the
Pennington Revenue Bond Indenture, to apply as a credit in respect of a
sinking fund obligation thereunder an amount of Pennington Series B Bonds
redeemed or purchased and delivered to the County Trustee, the corresponding
prepayment of this Bond in the same principal amount shall be credited against
the installment due on this Bond in the same year as such sinking fund
obligation.

    In each case where this Bond is to be prepaid in whole or in part as 
contemplated herein notice of not less than thirty (30) nor more than sixty
(60) days shall be given by first class mail postage prepaid to the holder of
record of this Bond unless such notice has been waived in writing by the
County Trustee.

    If an event of default as defined in the Indenture shall occur, the 
principal of this Bond may become and be declared due and payable in the
manner and with the effect provided in the Indenture. No holder of this Bond
shall have any right to institute any suit or proceeding for the foreclosure 
of the Indenture or for any other remedy thereunder, except to the extent and
in the manner set forth in the Indenture.

    This Bond shall be non-transferable except as required to effect (i) a 
transfer to any successor trustee under the Pennington Revenue Bond Indenture 
or (ii) a transfer after an event of default under the Pennington Revenue Bond 
Indenture in the course of the exercise of rights and remedies consequent upon 
such event of default. Subject to such restrictions, this Bond is transferable
by the registered holder hereof in person or by attorney duly authorized in
writing, at the office of Manufacturers Hanover Trust Company, New York, New
York, on registry books kept for such purpose at such office. No charge will
be made by the Company for any such transfer of this Bond. Before any transfer
of this Bond by the County Trustee or attorney duly authorized will be
recognized or given effect by the Company or the Trustee, the County Trustee
shall note hereon the date to which interest has been paid as well as the
amounts of all principal payments and prepayments hereon, and shall notify the
Company and the Trustee of the name and 
<PAGE>
 
                                      50
 







address of the transferee and shall afford the Company and the Trustee the 
opportunity of verifying the notation as to payment of interest and principal.

    No recourse shall be had for the payment of the principal of or the
interest on this Bond or for any claim based hereon or otherwise in respect
hereof or of the Indenture or of any agreement supplemental thereto against
any subscriber to the capital stock, incorporator or any past, present or
future stockholder, officer or director of the Company or of any predecessor
or successor corporation, either directly or through the Company or any
predecessor or successor corporation or any receiver or trustee in bankruptcy,
whether by virtue of any constitution or statute or rule of law or by the
enforcement of any assessment or penalty or stock subscription or otherwise,
all such liability, whether at common law or in equity or by statute or
constitution or otherwise, being, to the extent permitted by law, by the
acceptance and as a part of the consideration for the issuance hereof,
expressly waived and released by the registered holder hereof.

    This Bond shall not be valid or become obligatory for any purpose until the
certificate of authentication hereon shall have been signed by the Trustee, or
its successor as Trustee, under said Indenture.


    IN WITNESS WHEREOF, the Company has caused this Bond to be 
signed in its name by its President or one of its Vice-Presidents, and its 
corporate seal to be impressed or imprinted hereon and attested by its
Secretary or one of its Assistant Secretaries.

    Dated,
                                         BLACK HILLS POWER AND LIGHT COMPANY, 
 
                                         By.................................. 
Attest:                                                            President.

......................................
                            Secretary.
<PAGE>
 
                                      51
 






                       (FORM OF TRUSTEE'S CERTIFICATE)
 
    This is one of the Bonds, of the series designated therein, described in
the within mentioned Indenture.


                                         MANUFACTURERS HANOVER TRUST COMPANY, 
                                                                 as Trustee, 
 
                                         By .................................. 
                                                           Authorized Officer. 



                      SCHEDULE OF PAYMENTS OF PRINCIPAL
<TABLE>
<CAPTION>

 Amount of              Date              Date of          Authorized Official
Installment             Due               Payment               and Title 
- -----------             ----              -------          -------------------
<S>                     <C>               <C>              <C> 
</TABLE> 


 
                     SCHEDULE OF PREPAYMENTS OF PRINCIPAL


<TABLE>
<CAPTION>
Principal Amount                                           Authorized Official
    Prepaid                     Date Prepaid                    and Title
- ----------------                ------------               ------------------ 
<S>                             <C>                        <C>
</TABLE>
<PAGE>
 
                                      52
 
               SCHEDULE OF CREDITS OF PREPAYMENTS OF PRINCIPAL*


<TABLE>
<CAPTION>

                    Date of Installment 
Prepaid Principal        to which                            Authorized Official
Amount Credited          Credited         Date Credited           and Title
- -----------------   -------------------   -------------      -------------------
<S>                 <C>                   <C>                <C>
</TABLE>

________
*This Schedule to be completed only in the event that a prepayment is credited
against an installment maturing prior to the then last maturing unpaid
installment. Each prepayment is credited against the then last maturing unpaid
installment unless Pennington Series B Bonds redeemed or purchased and
delivered to the County Trustee are credited against a sinking fund obligation
under the Pennington Revenue Bond Indenture, whereupon a corresponding
prepayment in the same principal amount shall be credited against the
installment due in the same year as such sinking fund obligation and entered
in this Schedule. 

    (Notice: The within Bond may not be transferred until this schedule has
been verified by the Trustee.)

    SECTION 4.04. The single Bond of Series U is not prepayable prior 
to April 1, 1987, except that if, at any time, (1) the Company shall elect to
prepay installments payable under the Pennington Agreement No. 2 and to cause
the Pennington Series B Bonds to be redeemed upon the occurrence of an event 
specified in Section 9.1 of the Pennington Agreement No. 2 or (2) the Company 
shall be obligated to prepay installments payable under the Pennington 
Agreement No. 2 and to cause the Pennington Series B Bonds to be redeemed upon
the occurrence of an event specified in Section 9.2 of the Pennington 
Agreement No. 2, in any such event the Bonds of Series U shall be prepaid by 
the Company in whole, but not in part (except that upon the occurrence of an 
event specified in paragraph (c) of Section 9.2 of the Pennington Agreement
No. 2 the Bonds of Series U shall be prepaid in whole or in part) at 100% of the
unpaid principal amount thereof plus accrued interest to the prepayment date,
which date shall be the same date as the prepayment date under the Pennington
Agreement No. 2 determined as set forth in said Sections 9.1 and 9.2. The
principal amount of Bonds of Series U to be prepaid at any time upon the
occurrence of the event specified in para-
<PAGE>
 
                                      53




graph (c) of Section 9.2 of the Pennington Agreement No. 2 shall be the same 
as the principal amount of Pennington Series B Bonds to be redeemed as the 
result of the occurrence of such event.

    The single Bond of Series U shall be prepaid by the Company on or after 
April 1, 1987, in whole at any time or in part on any interest payment date, if
but only if the Company shall have elected to prepay installments under the 
Pennington Agreement No. 2 of like principal amount and to cause Pennington 
Series B Bonds of like principal amount to be redeemed on the prepayment date 
in accordance with the second paragraph of Section 3.01 of the Pennington 
Revenue Bond Indenture. Such prepayment shall be at the prepayment price 
determined in accordance with the following table plus accrued interest to
the prepayment date:
 
<TABLE>
<CAPTION>

     If the prepayment date is
     during the twelve months                            Prepayment
        beginning April 1                                  Price
     -------------------------                           ----------
<S>                                                      <C>
 
              1987......................................... 103    % 
              1988......................................... 102 1/2
              1989......................................... 102
              1990......................................... 101 1/2
              1991......................................... 101
              1992......................................... 100 1/2
              1993 and thereafter ......................... 100
</TABLE>

    Except to the extent that the Company shall at any time elect, pursuant to 
the provisions of the Pennington Revenue Bond Indenture, to apply as a credit
in respect of a sinking fund obligation thereunder an amount of Pennington
Series B Bonds redeemed or purchased and delivered to the County Trustee, any 
prepayment of only a part of the single Bond of Series U shall be in inverse 
order of the maturities of the several installment payments, e.g., all the
portion of the single Bond of Series U due on April 1, 2007, must be prepaid
before any installment having an earlier maturity date. In the event that the
Company at any time elects, pursuant to the provisions of the Pennington
Revenue Bond Indenture, to apply as a credit in respect of a sinking fund
obligation thereunder an amount of Pennington Series B Bonds redeemed or
purchased and delivered to the County Trustee, the corresponding prepayment of
the single Bond of Series U in the same principal amount shall be credited
against the installment due on the single Bond of Series U in the same year
as such sinking fund obligation. 
<PAGE>
 
                                      54






    If Bonds of Series U are to be redeemed in whole or in part as provided
in this Section 4.04, notice of redemption shall be given by first class mail,
postage prepaid, by or on behalf of the Company, not less than thirty (30) nor
more than sixty (60) days prior to the date of redemption, to the registered
holders of all Bonds so to be redeemed, at their respective addresses
appearing upon the book maintained by the Trustee pursuant to Section 2.10 of
the Original Indenture. Any notice which is mailed as herein provided shall be
conclusively presumed to have been properly and sufficiently given on the date
of such mailing, whether or not the registered holder or payee, as the case
may be, receives the notice. In case of any redemption of Bonds of such Series
by the Trustee pursuant to the provisions of the Indenture or any indenture
supplemental thereto, notice of redemption shall be given in a similar manner
by the Trustee. Notwithstanding any provision of Article Ten of the Original
Indenture, no publication of notice of redemption of any Bond of Series U
shall be required by the Indenture.


    Except as provided in the immediately preceding paragraph, the provisions 
of Article Ten of the Original Indenture shall in all respects apply to any 
redemption to which this Section 4.04 applies. Nothing in this Section 4.04 
contained shall affect the manner of giving notice of the redemption of Bonds
of the Company of any series other than Bonds of Series U.

    Notwithstanding the provisions of the Original Indenture (including 
particularly Section 8.08 (b) thereof), the single Bond of Series U is subject
to redemption prior to maturity only upon the events and as set forth in this
Section 4.04. 

                                 ARTICLE FIVE
 
                           MISCELLANEOUS PROVISIONS
 
 
    SECTION 5.01. The Company, by the execution hereof, acknowledges 
that a true copy of this Supplemental Indenture has been delivered to and
received by it. 

    SECTION 5.02. Except as heretofore supplemented and amended and as 
amended by this Supplemental Indenture, all the provisions, terms and
conditions of the Original Indenture shall continue in full force and effect. 
<PAGE>
 
                                      55
 





    SECTION 5.03. This Supplemental Indenture may be executed in several 
counterparts, all or any of which may be treated for all purposes as one 
original and shall constitute and be one and the same instrument.
 
    IN WITNESS WHEREOF, BLACK HILLS POWER AND LIGHT COMPANY, party hereto of
the first part, has caused this Supplemental Indenture to be executed on its
behalf by the Chairman of its Board, its President or one of its Vice
Presidents and its corporate seal to be hereto affixed and to be attested by
its Secretary or an Assistant Secretary, and MANUFACTURERS HANOVER TRUST
COMPANY, party hereto of the second part, in evidence of its acceptance of the
trust hereby created, has caused this Supplemental Indenture to be executed on
its behalf by one of its Vice Presidents or Assistant Vice Presidents and its
corporate seal to be hereto affixed and to be attested by its Secretary or an
Assistant Secretary, all as of the day and year first above written.
 
                                          BLACK HILLS POWER AND LIGHT COMPANY,

[CORPORATE SEAL]
 
                                                   NEIL G. SIMPSON
                                                         Chairman of the Board.
Attest:
 
GEORGE T. LOCKE
                       Secretary.
 
Signed, sealed and delivered by
 BLACK HILLS POWER AND LIGHT
 COMPANY in the presence of:
  JAMES H. WILSON
  DAVID E. MORRILL
 
                                            MANUFACTURERS HANOVER TRUST COMPANY,
[CORPORATE SEAL]
 
                                                    E. F. COCKINGS
                                                      Assistant Vice President. 
Attest: 
 
 
K. C. KELLY
                     Assistant Secretary.
 
Signed, sealed and delivered by
 MANUFACTURERS HANOVER TRUST
 COMPANY in the presence of:
   F. J. GRIPPO
   JAMES M. FOLEY
<PAGE>
 
                                      56
 







STATE OF NEW YORK   )
                    )   SS.:
COUNTY OF NEW YORK  )

    On this 20th (lay of April, 1977, before me, THERESA CERTO, the 
undersigned officer, personally appeared NEIL G. SIMPSON, to me personally 
known, who acknowledged himself to be, and being by me duly sworn, did say 
that he is Chairman of the Board of BLACK HILLS POWER AND LIGHT COMPANY, a
corporation, and that the seal affixed to the foregoing instrument is the
corporate seal of said corporation and that said instrument was executed by,
and signed in the name of, the corporation, by him, as such Chairman of the
Board and sealed in behalf of the corporation by authority of its Board of
Directors for the purposes therein contained, and the said NEIL G. SIMPSON
acknowledged the same as the free act and deed of said corporation.

    IN WITNESS WHEREOF, I hereunto set my hand and official seal. 
 
 
                                                    Notary Public
[NOTARIAL SEAL]

                                                    Notary Public
                                                    THERESA CERTO
                                           Notary Public, State of New York
                                                   No. 01CE4624382
                                              Qualified in Queens County
                                           Commission Expires March 30, 1978
 
My commission expires
<PAGE>
 
                                      57







STATE OF NEW YORK   )
                    )   SS.:
COUNTY OF NEW YORK  )

    On this 18th day of April, 1977, before me, MARY LEONARDI, the 
undersigned officer, personally appeared E. F. COCKINGS, to me personally 
known, who acknowledged himself to be, and being by me duly sworn, did say 
that he is Assistant Vice President of MANUFACTURERS HANOVER TRUST COMPANY, a
corporation, and that the seal affixed to the foregoing instrument is the
corporate seal of said corporation and that said instrument was executed by,
and signed in the name of, the corporation, by him, as such Assistant Vice 
President and sealed in behalf of the corporation by authority of its Board of 
Directors for the purposes therein contained, and the said E. F. COCKINGS 
acknowledged the same as the free act and deed of said corporation.

    IN WITNESS WHEREOF, I hereunto set my hand and official seal.
 
                                                 Notary Public
                                                 MARY LEONARDI
[NOTARIAL Seal]

                                                 MARY LEONARDI
                                        Notary Public, State of New York
                                                 No. 40-7495520
                                           Qualified in Putnam County
                                       Certificate filed in New York County
                                         Commission Expires March 30,1978
 

<PAGE>
 
                                                                [CONFORMED COPY]
================================================================================







                     BLACK HILLS POWER AND LIGHT COMPANY

                                      TO

                     MANUFACTURERS HANOVER TRUST COMPANY,
                                                      As Trustee





                                  ----------


                     TWENTY FIRST SUPPLEMENTAL INDENTURE

                           Dated as of June 1, 1977


                                  ----------




                  Supplemental to Indenture of Mortgage and
                 Deed of Trust Dated as of September 1, 1941








================================================================================
 
<PAGE>
 
    TWENTY FIRST SUPPLEMENTAL INDENTURE, dated as of the 1st day of June, 1977
between BLACK HILLS POWER AND LIGHT COMPANY, a corporation organized and
existing under the laws of the State of South Dakota (herein called the
"Company"), party of the first part and MANUFACTURERS HANOVER TRUST COMPANY,
a corporation organized and existing under the laws of the State of New York,
as Trustee under the Indenture hereinafter mentioned (hereinafter called the
"Trustee"), party of the second part.
 
    WHEREAS, in order to secure an authorized issue of First Mortgage Bonds of
the Company, the Company has executed and delivered an Indenture of Mortgage and
Deed of Trust to Central Hanover Bank and Trust Company (subsequently known as
The Hanover Bank), as Trustee, dated September 1, 1941, hereinafter referred to
as the "Original Indenture", and has also executed and delivered to said
Trustee and to Manufacturers Hanover Trust Company (which on September 8, 1961
became the Trustee under the Original Indenture, as theretofore supplemented and
amended, by virtue of the merger of said The Hanover Bank into Manufacturers
Trust Company, under said name Manufacturers Hanover Trust Company), as Trustee,
various Supplemental Indentures supplementing and/or modifying the Original
Indenture, respectively dated as of July 15, 1945, January 15, 1948, January 15,
1949, March 1, 1950, March 1, 1952, July 1, 1956, May 1, 1957, May 1, 1959,
April 1, 1960, August 1, 1960, June 1, 1961, October 1, 1962, May 1, 1963, June
1, 1969, June 15, 1974, August 1, 1974, July 15, 1975, May 1, 1976, February 15,
1977, and April 1, 1977 and the Company has also executed and delivered to said
The Hanover Bank, as Trustee, Subordination Agreements dated December 29, 1950
and September 19, 1955, also supplementing the Original Indenture which, as
supplemented and amended by said twenty Supplemental Indentures and said
Subordination Agreements, is hereinafter referred to as the "Indenture"; and
 
    WHEREAS, pursuant to the provisions of the Indenture, First Mortgage Bonds
have been duly issued and are presently outstanding and secured by the
Indenture as follows: Series D, 3 5/8%, due January 15, 1979, Series E, 3 1/4%
due March 1, 1980, Series F, 4 1/8%, due March 1, 1982, Series G, 4 1/4%, due
July 1, 1986, Series H, 5 1/4%, due May 15, 1987, Series I, 5 1/8%, due May 1,
1989, Series J, 5 3/4%, due April 1,
 
<PAGE>
 
                                       2





1990, Series K, 5 5/8%, due August 1, 1990, Series L, 5 3/8%, due June 1, 1991,
Series M, 5%, due October 1, 1992, Series N, 4 3/4%, due May 1, 1993, Series O,
8.05%, due June 1, 1999, Series P, 10.75%, due August 1, 2004, Series Q,
11 1/4%, due July 15, 1985, Series R, 6 5/8%, due April 1, 2007, Series S,
6 5/8%, due April 1, 2007, Series T, 6 5/8%, due April 1, 2007 and Series U,
6 5/8%, due April 1, 2007; and
 
    WHEREAS, the Company has entered into an Agreement dated as of June 1, 1977
(the "Campbell Agreement") with Campbell County, Wyoming ("Campbell") a
political subdivision of the State of Wyoming, for the acquisition,
construction and improvement of certain facilities for air and water pollution
control purposes at the Neil Simpson Electric Generating Plant of the Company
and an Agreement dated as of June 1, 1977 (the "Weston Agreement") with
Weston County, Wyoming ("Weston") also a political subdivision of the State
of Wyoming, for the acquisition, construction and improvement of certain
facilities for air and water pollution control purposes at the Osage Electric
Generating Plant of the Company and said pollution control facilities will be
financed through the issuance by each County of its respective Pollution
Control Revenue Bonds (Black Hills Power and Light Company Project)
Collateralized 1977 Series A (hereinafter respectively, the "Campbell Series A
Bonds" and the "Weston Series A Bonds" and, collectively, the "County
Series A Bonds") and in connection therewith the Company will execute and
deliver its First Mortgage Bonds of Series V and Series W provided for and
created by this Supplemental Indenture which will be pledged by Campbell and
Weston, respectively, to the trustee (the "County Trustee") under each
County's Indenture of Trust dated as of June 1, 1977 (hereinafter,
respectively, the "Campbell Pollution Control Indenture" and the "Weston
Pollution Control Indenture" and, collectively, the "Pollution Control
Indentures") providing for the issuance of the related County's Series A Bonds
with respect to said pollution control facilities, as security for the payment
of the principal of and premium, if any, and interest on the related County
Series A Bonds; and
 
    WHEREAS, as permitted by the Indenture, the Company by resolutions of its
Board of Directors duly adopted, has determined to create a new series of bonds
to be known as its "First Mortgage Bonds,
<PAGE>
 
                                       3

Series V" (hereinafter sometimes called the "Bonds of Series V") and "First
Mortgage Bonds, Series W" (hereinafter sometimes called the "Bonds of Series
W"), each in the form, having the characteristics and being entitled to the
benefits as in this Supplemental Indenture provided; and
 
    WHEREAS, the Company, in the exercise of the powers and authority conferred
upon and reserved to it under and by virtue of the provisions of the Original
Indenture, and particularly the provisions contained in Articles Two and
Seventeen thereof, and pursuant to appropriate resolutions of its Board of
Directors, has duly resolved and determined to make, execute and deliver to the
Trustee a Supplemental Indenture in the form hereof for the purposes herein
provided; and
 
    WHEREAS, the Company is about to issue, under the Indenture, as hereby
amended and supplemented, its Bonds of Series V in the principal amount of
$1,55O,000 in fully registered form and without coupons and its Bonds of Series
W in the principal amount of $2,850,000 in fully registered form and without
coupons payable to the County Trustee, as assignee of Campbell and Weston under
the related Pollution Control Indenture. Said Bonds of Series V and Series W
will otherwise be non-transferable except as required to effect (i) a transfer
after an event of default under the related Pollution Control Indenture in the
course of the exercise of rights and remedies consequent upon such event of
default, or (ii) a transfer to a successor trustee under the related Pollution
Control Indenture; and
 
    WHEREAS, all conditions and requirements necessary to make this
Supplemental Indenture a valid, binding and legal instrument in accordance with
its terms have been done, performed and fulfilled, and the execution and
delivery hereof have been in all respects duly authorized:
 
    NOW, THEREFORE, THIS INDENTURE WITNESSETH: That the Company, in
consideration of the premises and of one dollar to it duly paid by the Trustee
at or before the ensealing and delivery of these presents, the receipt whereof
is hereby acknowledged, and of other good and valuable consideration, in order
to better secure the payment
<PAGE>
 
                                       4

both of the principal of and interest on all Bonds issued under the Indenture
and that may be issued under this or any other indentures supplemental thereto,
according to their tenor and effect, and the performance by the Company of all
the covenants and conditions herein and therein contained, and in order to
establish the terms of the Bonds of Series V and Series W and to amend certain
provisions of the Indenture, hereby further covenants and agrees to and with
the Trustee and its successors in the trust under the Indenture for the benefit
of all those who shall from time to time hold the Bonds of Series V and Series
W as follows:
 
                                 ARTICLE ONE
 
                           BONDS OF SERIES V, 6.85%
 
    SECTION 1.01. There is hereby created a series of Bonds, known as and
entitled "First Mortgage Bonds Series V, 6.85%", due June 1, 2007, and the
form thereof shall be as provided in this Supplemental Indenture.
 
    SECTION 1.02. The Bonds of Series V shall be evidenced by a single
registered bond in the principal amount and denomination of One Million Five
Hundred Fifty Thousand Dollars ($1,550,000) finally due June 1, 2007 but
payable in installments as follows:
 
<TABLE>
<CAPTION>
       June 1           Principal         June 1          Principal
    Of the Year          Amount        Of the Year         Amount
    -----------         ---------      -----------        ---------
    <S>                 <C>            <C>                <C>
        1993 ........... $75,000           2001 ......... $ 75,000
        1994 ...........  75,000           2002 .........   75,000
        1995 ...........  75,000           2003 .........   75,000
        1996 ...........  75,000           2004 .........   75,000
        1997 ...........  75,000           2005 .........   75,000
        1998 ...........  75,000           2006 .........   75,000
        1999 ...........  75,000           2007 .........  500,000
        2000 ...........  75,000
</TABLE>
 
The Bonds of Series V shall hear interest on the unpaid principal balance at
the rate of 6.85% per annum payable semi-annnally on December 1 and June 1
commencing December 1, 1977 until due and payable and shall bear interest at
the rate of 6% per annum on any overdue principal and on any overdue
installment of interest (to the
<PAGE>
 
                                       5

extent that payment of such interest is enforceable under applicable law). June
1, 1977 shall be the date of commencement of the first interest period for such
Bonds. All such bonds shall be dated as provided in Section 2.05 of the
Original Indenture.
 
    The single Bond of Series V shall be lettered V and numbered 1 and shall,
upon issuance, be delivered by the Company to and registered in the name of the
County Trustee and shall be transferable only as required to effect an
assignment thereof to a successor County Trustee. Bonds issued upon transfer
shall be lettered V and numbered from 2 upwards and issued in the same
$1,550,000 denomination but all payments of principal theretofore made on the
Bonds of Series V shall be duly noted thereon by the Trustee.
 
    It is expected that the Company, pursuant to the Campbell Agreement, will
furnish directly to the County Trustee at its principal corporate trust office
all funds required for any and all payments of principal of, and interest and
premium on, the Campbell Series A Bonds (or that in lieu of any such payment
the Company or Campbell will deposit with the County Trustee Campbell Series A
Bonds for cancellation) and that corresponding payments of interest and of
installments of principal (including premiums if appropriate) on the Bonds of
Series V will automatically be effected in accordance with the provisions of
the Campbell Agreement. Whenever payment or provision therefor has been made in
respect of the principal, premium, if any, or interest on all or any portion of
the Campbell Series A Bonds in accordance with the Campbell Pollution Control
Indenture, the corresponding principal installment, premium, if any, or
interest on the Bonds of Series V shall be deemed paid to the extent such
payment or provision therefor has been made. Unless payment then is or has been
made as above provided, payment of the principal of, and premium, if any, and
interest on the single Bond of Series V shall be made to the County Trustee for
the account of Campbell in funds immediately available at said office of the
County Trustee, in each case on or prior to the due date for such payment. The
Trustee may at any and all times conclusively assume that the obligation of the
Company to make payments with respect to the principal of and premium, if any,
and interest on the Bonds of Series V, so far as such payments at the time have
become due, has been fully satisfied and discharged unless and until the
Trustee shall have received a written
<PAGE>
 
                                       6

notice from the County Trustee signed by one of its officers stating (i) that
timely payment of principal of, or premium or interest on, Bonds of Series V
has not been made, (ii) that the Company is in arrears as to the payments
required to be made by it to the County Trustee pursuant to the Campbell
Agreement after giving effect to any available moneys in the Bond Fund provided
by the Campbell Pollution Control Indenture and (iii) the amount of the
arrearage.
 
    The County Trustee, by acceptance of the single Bond of Series V, shall
agree to make prompt notation thereon of all payments and prepayments on
account of principal thereof made or occurring under any provisions of the
Campbell Agreement or of this Supplemental Indenture, and to surrender said
Bond to the Trustee upon final payment thereof. If and to the extent payment of
the Campbell Series A Bonds is deemed to have been made pursuant to the terms
of the Campbell Pollution Control Indenture, payment of the Bonds of Series V
shall be deemed to have been made to the same extent.
 
    The Trustee is hereby appointed Registrar in respect of the Bonds of Series
V, and the principal corporate trust office of the Trustee in the Borough of
Manhattan, The City of New York, is hereby designated as the office or agency
of the Company in said Borough where notices or demands in respect of Bonds of
Series V may be served.
 
    SECTION 1.03. The text of the Bonds of Series V and the certificate of
authentication of the Trustee to be executed thereon, are to be substantially
in the following forms, respectively:
 
No. V ..........                                                      $1,500,000
 
                     BLACK HILLS POWER AND LIGHT COMPANY
 
            FIRST MORTGAGE BOND SERIES V, 6.85%, DUE JUNE 1, 2007
 
    BLACK HILLS POWER AND LIGHT COMPANY (hereinafter called the "Company"), a
corporation organized and existing under the laws of the State of South Dakota,
for value received, hereby promises to pay to                          
as trustee or its successor in trust (the "County Trustee"), under an
Indenture of Trust dated as of June 1, 1977 (the "Campbell Pollution Control
Indenture") between Campbell County, Wyoming and said County
<PAGE>
 
                                       7


Trustee, ONES MILLION FIVE HUNDRED FIFTY THOUSAND DOLLARS, ($1,550,000) payable
in installments as follows:
 
<TABLE>
<CAPTION>
       June 1           Principal         June 1          Principal
    Of the Year          Amount        Of the Year         Amount
    -----------         ---------      -----------        ---------
    <S>                 <C>            <C>                <C>
        1993 ........... $75,000           2001 ......... $ 75,000
        1994 ...........  75,000           2002 .........   75,000
        1995 ...........  75,000           2003 .........   75,000
        1996 ...........  75,000           2004 .........   75,000
        1997 ...........  75,000           2005 .........   75,000
        1998 ...........  75,000           2006 .........   75,000
        1999 ...........  75,000           2007 .........  500,000
        2000 ...........  75,000
</TABLE>
 
together with interest thereon from June 1, 1977 on the unpaid principal amount
of this Bond at the rate of 6.85% per annum until due and payable, such
interest to be paid semi-annually in arrears on December 1 and June 1 in each
year. Overdue principal and overdue installments of interest shall bear
interest at the rate of 6% per annum (to the extent that payment of such
interest is enforceable under applicable law).
 
    This Bond is one of an authorized issue of Bonds of the Company known as
its "First Mortgage Bonds", issued and to be issued in one or more series
under, and all equally and ratably secured (except as any sinking,
amortization, improvement, renewal or other analogous fund, established in
accordance with the provisions of the Indenture hereinafter mentioned, may
afford additional security for the Bonds of any particular series) by, an
Indenture of Mortgage and Deed of Trust dated as of September 1, 1941 as
supplemented and amended by Supplemental Indentures dated as of July 15, 1945,
January 15, 1948, January 15, 1949, March 1, 1950, March 1, 1952, July 1, 1956,
May 1, 1957, May 1, 1959, April 1, 1960, August 1, 1960, June 1, 1961, October
1, 1962, May 1, 1963, June 1, 1969, June 15, 1974, August 1, 1974, July 15,
1975, May 1, 1976, February 15, 1977 and April 1, 1977 and subordination
agreements dated December 29, 1950 and September 19, 1955 (hereinafter
collectively called the "Indenture"), executed by the Company to
Manufacturers Hanover Trust Company (herein called the "Trustee"), as
Trustee, to which Indenture and all indentures supplemental thereto reference
is hereby made for a description of the properties mortgaged and pledged, the
nature and extent of the
<PAGE>
 
                                       8

security, the rights of the holders of said Bonds and the coupons appurtenant
to coupon Bonds and of the Trustee and of the Company in respect of such
security, and the terms and conditions upon which said Bonds are and are to be
issued and secured.
 
    To the extent permitted by the Indenture and as provided therein, with the
consent of the Company and upon the written consent or affirmative vote of at
least sixty-six and two-thirds per cent. in principal amount of the Bonds then
outstanding and entitled to consent, and of not less than sixty-six and two-
thirds per cent. in principal amount of the Bonds then outstanding and entitled
to consent of each series affected thereby in case one or more but less than
all of the series of Bonds issued under the Indenture are so affected, the
rights and obligations of the Company and of the holders of Bonds and coupons
appurtenant to coupon Bonds, and the terms and provisions of the Indenture and
of any instrument supplemental thereto may be modified from time to time,
provided that no such modification or alteration shall be made which would
postpone the date fixed herein or in the coupons or in the Indenture for the
payment of the principal of, or any installment of interest on, the Bonds, or
reduce the principal of, or the rate of interest payable on, the Bonds, or
reduce the percentage of the principal amount of Bonds the consent of which is
required for the authorization of any such modification or alteration, or which
would modify, without the written consent of the Trustee, the rights, duties or
immunities of the Trustee.
 
    As provided in said Indenture, said Bonds are issuable in series which may
vary as in said Indenture provided or permitted. This Bond is one of a series
of Bonds entitled "First Mortgage Bonds Series V".
 
    Campbell County, Wyoming (the "County") is issuing and selling, or has
issued and sold, its Pollution Control Revenue Bonds (Black Hills Power and
Light Company Project) Collateralized 1977 Series A (hereinafter called the
"Campbell Series A Bonds") under and pursuant to the Campbell Pollution
Control Indenture. This Bond of Series V is issued as a means of securing
repayment of the loan Of the proceeds of the Campbell Series A Bonds to the
Company to finance the acquisition, construction and improvement of certain
pollution control facilities pursuant to a Financing Agreement dated as of June
1, 1977 (the "Campbell Agreement"). It is expected that the Company will make
all payments of principal of, and interest and premium on, the Campbell Series
A Bonds directly to the County
<PAGE>
 
                                       9

Trustee (or that in lieu thereof the Company or Campbell will deposit with the
County Trustee Campbell Series A Bonds for cancellation). All such payments
shall automatically constitute corresponding, payments on the Bonds of Series V
in accordance with the provisions of the Campbell Agreement. The holder of this
Bond by acceptance hereof agrees that whenever any payment or prepayment on
account of the principal of this Bond is made or occurs under any provision of
the Indenture, the Campbell Agreement or the Campbell Pollution Control
Indenture, the holder hereof shall promptly note on the Schedule of Payments of
Principal, the Schedule of Prepayments of Principal or the Schedule of Credits
of Prepayments of Principal the date and amount of each such payment or
prepayment of principal, and shall promptly notify the Trustee of the amount of
each such payment and that the notation of payment has been duly made, and
further agrees to surrender this Bond to the Trustee for cancellation when all
principal of, premium, if any, and interest on this Bond shall have been duly
paid. The holder of this Bond further agrees that if payment of the Campbell
Series A Bonds is deemed to have been made pursuant to the provisions of the
Campbell Pollution Control Indenture, payment of this Bond shall be deemed to
have been made.
 
    Unless payment then is or has been made pursuant to the foregoing
paragraph, the principal of and premium, if any, and interest on this Bond will
be paid in lawful money of the United States of America and will be payable at
the principal corporate trust office of the County Trustee to the County
Trustee for the account of Campbell in immediately available funds at said
office of the County Trustee, in each case on or prior to the due date for such
payment. The holder of this Bond by acceptance hereof agrees that any such
payment of principal, premium or interest on this Bond shall be credited as and
used to make a corresponding, payment of principal, premium or interest on the
Campbell Series A Bonds.
 
    This Bond is not prepayable prior to June 1, 1987, except that if at any
time (1) the Company shall elect to prepay installments payable under the
Campbell Agreement and to cause the Campbell Series A Bonds to be redeemed upon
the occurrence of an event specified in Section 8.1 of the Campbell Agreement
or (2) the Company shall be obligated to prepay installments payable under the
Campbell Agreement and to cause the Campbell Series A Bonds to be redeemed upon
the occurrence of an event specified in Section 8.2 of the Campbell
<PAGE>
 
                                       10

Agreement, in any such event this Bond shall be prepaid by the Company in
whole, but not in part (except that upon the occurrence of an event specified
in paragraph (c) of Section 8.2 of the Campbell Agreement the Bond shall be
prepaid in whole or in part) at 100% of the unpaid principal amount thereof
plus accrued interest to the prepayment date, which date shall be the same date
as the prepayment date under the Campbell Agreement determined as set forth in
said Sections 8.1 and 8.2. The principal amount of Bonds of Series V to be
prepaid at any time upon the occurrence of the event specified in paragraph (c)
of Section 8.2 of the Campbell Agreement shall be the same as the principal
amount of Campbell Series A Bonds to be redeemed as the result of the
occurrence of such event.
 
    This Bond shall be prepaid by the Company on or after June 1, 1987, in
whole at any time or in part on any interest payment date, if but only if the
Company shall have elected to prepay installments under the Campbell Agreement
of like principal amount and to cause Campbell Series A Bonds of like principal
amount to be redeemed on the prepayment date in accordance with the second
paragraph of Section 3.01 of the Campbell Pollution Control Indenture. Such
prepayment shall be at the prepayment price determined in accordance with the
following table plus accrued interest to the prepayment date:
 
<TABLE>
<CAPTION>
 If prepayment date is during
twelve-month-period beginning                         Prepayment
           June l                                        Price
- -----------------------------                         ----------
<S>                                                   <C>
            1987 ..................................... 103    %
            1988 ..................................... 102 1/2
            1989 ..................................... 102
            1990 ..................................... 101 1/2
            1991 ..................................... 101
            1992 ..................................... 100 1/2
            1993 and thereafter ...................... 100
</TABLE>
 
    Except to the extent that the Company shall at any time elect, pursuant to
the provisions of the Campbell Pollution Control Indenture, to apply as a credit
in respect of a sinking fund obligation thereunder an amount of Campbell Series
A Bonds redeemed or purchased and delivered to the County Trustee, any
prepayment of only a part of this Bond shall be in inverse order of the
maturities of the several installment payments, e.g., all the portion of this
Bond due on June 1, 2007, must be prepaid before any installment having an
earlier maturity
<PAGE>
 
                                       11

date. In the event that the Company at any time elects, pursuant to the
provisions of the Campbell Pollution Control Indenture, to apply as a credit in
respect of a sinking fund obligation thereunder an amount of Campbell Series A
Bonds redeemed or purchased and delivered to the County Trustee, the
corresponding prepayment of this Bond in the same principal amount shall be
credited against the installment due on this Bond in the same year as such
sinking fund obligation.
 
    In each case where this Bond is to be prepaid in whole or in part as
contemplated herein notice of not less than thirty (30) nor more than sixty
(60) days shall be given by first class mail postage prepaid to the holder of
record of this Bond unless such notice has been waived in writing by the County
Trustee.
 
    If an event of default as defined in the Indenture shall occur, the
principal of this Bond may become and be declared due and payable in the manner
and with the effect provided in the Indenture. No holder of this Bond shall
have any right to institute any suit or proceeding for the foreclosure of the
Indenture or for any other remedy thereunder, except to the extent and in the
manner set forth in the Indenture.
 
    This Bond shall be non-transferable except as required to effect (i) a
transfer to any successor trustee under the Campbell Pollution Control
Indenture or (ii) a transfer after an event of default under the Campbell
Pollution Control Indenture in the course of the exercise of rights and
remedies consequent upon such event of default. Subject to such restrictions,
this Bond is transferable by the registered holder hereof in person or by
attorney duly authorized in writing, at the office of Manufacturers Hanover
Trust Company, New York, New York, on registry books kept for such purpose at
such office. No charge will be made by the Company for any such transfer of
this Bond. Before any transfer of this Bond by the County Trustee or attorney
duly authorized will be recognized or given effect by the Company or the
Trustee, the County Trustee shall note hereon the date to which interest has
been paid as well as the amounts of all principal payments and prepayments
hereon, and shall notify the Company and the Trustee of the name and address of
the transferee and shall afford the Company and the Trustee the opportunity of
verifying the notation as to payment of interest and principal.
 
    No recourse shall be had for the payment of the principal of or the
interest on this Bond or for any claim based hereon or otherwise in respect
hereof or of the Indenture or of any agreement supplemental
<PAGE>
 
                                       12

thereto against any subscriber to the capital stock, incorporator or any past,
present or future stockholder, officer or director of the Company or of any
predecessor or successor corporation, either directly or through the Company or
any predecessor or successor corporation or any receiver or trustee in
bankruptcy, whether by virtue of any constitution or statute or rule of law or
by the enforcement of any assessment or penalty or stock subscription or
otherwise, all such liability, whether at common law or in equity or by statute
or constitution or otherwise, being, to the extent permitted by law, by the
acceptance and as a part of the consideration for the issuance hereof,
expressly waived and released by the registered holder hereof.
 
    This Bond shall not be valid or become obligatory for any purpose until the
certificate of authentication hereon shall have been signed by the Trustee, or
its successor as Trustee, under said Indenture.
 
    IN WITNESS WHEREOF, the Company has caused this Bond to be signed in its
name by its President or one of its Vice-Presidents, and its corporate seal to
be impressed or imprinted hereon and attested by its Secretary or one of its
Assistant Secretaries.
 
    Dated,
 
                                            BLACK HILLS POWER AND LIGHT COMPANY,
 
                                               By ..............................
                                                                    President.
 
Attest:
 
    ....................................
                            Secretary.
 
                       (FORM OF TRUSTEE'S CERTIFICATE)
 
    This is one of the Bonds, of the series designated therein, described in
the within mentioned Indenture.
 
                                            MANUFACTURERS HANOVER TRUST COMPANY,
                                                                     as Trustee,
 
                                              By ...............................
                                                           Authorized Officer.
<PAGE>
 
                                       13

                      SCHEDULE OF PAYMENTS OF PRINCIPAL
<TABLE>
<CAPTION>
 
 Amount of          Date             Date of             Authorized Official
Installment         Due              Payment                  and Title
- -----------         ----             -------             -------------------
<S>                 <C>              <S>                 <S>
</TABLE>
 
 
 
 
 
                     SCHEDULE OF PREPAYMENTS OF PRINCIPAL
<TABLE>
<CAPTION>
 
        Principal Amount                        Authorized Official
             Prepaid         Date Prepaid            and Title
        ----------------     ------------       -------------------
        <S>                  <C>                <C>
</TABLE>
 
 
 
 
 
               SCHEDULE OF CREDITS OF PREPAYMENTS OF PRINCIPAL*
<TABLE>
<CAPTION>
 
                      Date of Installment
Prepaid Principal           to which                         Authorized Official
 Amount Credited            Credited        Date Credited         and Title
- -----------------     -------------------   -------------    -------------------
<S>                   <C>                   <C>              <C>
</TABLE>
 
 
 
    *This Schedule to be completed only in the event that a prepayment is
credited against an installment maturing prior to the then last maturing unpaid
installment. Each prepayment is credited against the then last maturing unpaid
installment unless Campbell Series A Bonds redeemed or purchased and delivered
to the County Trustee are credited against a sinking fund obligation under the
Campbell Pollution Control Indenture, whereupon a corresponding prepayment in
the same principal amount shall be credited against the installment due in the
same year as such sinking fund obligation and entered in this Schedule.
 
    (NOTICE: The within Bond may not be transferred until this schedule has been
verified by the Trustee.)
<PAGE>
 
                                       14

    Section 1.04. The single Bond of Series V is not prepayable prior to June 1,
1987, except that if, at any time, (1) the Company shall elect to prepay
installments payable under the Campbell Agreement and to cause the Campbell
Series A Bonds to be redeemed upon the occurrence of an event specified in
Section 8.1 of the Campbell Agreement or (2) the Company shall be obligated to
prepay installments payable under the Campbell Agreement and to cause the
Campbell Series A Bonds to be redeemed upon the occurrence of an event specified
in Section 8.2 of the Campbell Agreement, in any such event the Bonds of Series
V shall be prepaid by the Company in whole, but not in part (except that upon
the occurrence of an event specified in paragraph (c) of Section 8.2 of the
Campbell Agreement the Bonds of Series V shall be prepaid in whole or in part)
at 100% of the unpaid principal amount thereof plus accrued interest to the
prepayment date, which date shall be the same date as the prepayment date under
the Campbell Agreement determined as set forth in said Sections 8.1 and 8.2. The
principal amount of Bonds of Series V to be prepaid at any time upon the
occurrence of the event specified in paragraph (c) of Section 8.2 of the
Campbell Agreement shall be the same as the principal amount of Campbell Series
A Bonds to be redeemed as the result of the occurrence of such event.
 
    The single Bond of Series V shall be prepaid by the Company on or after June
1, 1987, in whole at any time or in part on any interest payment date, if but
only if the Company shall have elected to prepay installments under the Campbell
Agreement of like principal amount and to cause Campbell Series A Bonds of like
principal amount to be redeemed on the prepayment date in accordance with the
second paragraph of Section 3.01 of the Campbell Pollution Control Indenture.
Such prepayment shall be at the prepayment price determined in accordance with
the following table plus accrued interest to the prepayment date:
 
<TABLE>
<CAPTION>
If the prepayment date is
 during the twelve months                                Prepayment
     beginning June 1                                       Price
- -------------------------                                ----------
<S>                                                      <C>
           1987 ......................................... 103    %
           1988 ......................................... 102 1/2
           1989 ......................................... 102
           1990 ......................................... 101 1/2
           1991 ......................................... 101
           1992 ......................................... 100 1/2
           1993 and thereafter........................... 100
</TABLE>
<PAGE>
 
                                       15

    Except to the extent that the Company shall at any time elect, pursuant to
the provisions of the Campbell Pollution Control Indenture, to apply as a credit
in respect of a sinking fund obligation thereunder an amount of Campbell Series
A Bonds redeemed or purchased and delivered to the County Trustee, any
prepayment of only a part of the single Bond of Series V shall be in inverse
order of the maturities of the several installment payments, e.g., all the
portion of the single Bond of Series V due on June l, 2007, must be prepaid
before any installment having an earlier maturity date. In the event that the
Company at any time elects, pursuant to the provisions of the Campbell Pollution
Control Indenture, to apply as a credit in respect of a sinking fund obligation
thereunder an amount of Campbell Series A Bonds redeemed or purchased and
delivered to the County Trustee, the corresponding prepayment of the single Bond
of Series V in the same principal amount shall be credited against the
installment due on the single Bond of Series V in the same year as such sinking
fund obligation.
 
    If Bonds of Series V are to be redeemed in whole or in part as provided in
this Section 1.04, notice of redemption shall be given by first class mail,
postage prepaid, by or on behalf of the Company, not less than thirty (30) nor
more than sixty (60) days prior to the date of redemption, to the registered
holders of all Bonds so to be redeemed, at their respective addresses appearing
upon the books maintained by the Trustee pursuant to Section 2.10 of the
Original Indenture. Any notice which is mailed as herein provided shall be
conclusively presumed to have been properly and sufficiently given on the date
of such mailing, whether or not the registered holder or payee, as the case may
be, receives the notice. In case of any redemption of Bonds of such Series by
the Trustee pursuant to the provisions of the Indenture or any indenture
supplemental thereto, notice of redemption shall be given in a similar manner
by the Trustee. Notwithstanding any provision of Article Ten of the Original
Indenture, no publication of notice of redemption of any Bonds of Series V shall
be required by the Indenture.
 
    Except as provided in the immediately preceding paragraph, the provisions of
Article Ten of the Original Indenture shall in all respects apply to any
redemption to which this Section 1.04 applies. Nothing in this Section 1.04
contained shall affect the manner of giving notice of
<PAGE>
 
                                       16

the redemption of Bonds of the Company of any series other than Bonds of Series
V.
 
    Notwithstanding the provisions of the Original Indenture (including
particularly Section 8.08(b) thereof), the single Bond of Series V is subject to
redemption prior to maturity only upon the events and as set forth in this
Section 1.04.
 
                                  ARTICLE TWO
 
                           BONDS OF SERIES W, 6.85%
 
    SECTION 2.01. There is hereby created a series of Bonds, known as and
entitled "First Mortgage Bonds Series W, 6.85%", due June 1, 2007, and the
form thereof shall be as provided in this Supplemental Indenture.
 
    SECTION 2.02. The Bonds of Series W shall be evidenced by a single
registered bond in the principal amount and denomination of Two Million Eight
Hundred Fifty Thousand Dollars ($2,850,000) finally due June 1, 2007 but payable
in installments as follows:
 
<TABLE>
<CAPTION>
  
      June 1           Principal         June 1          Principal
    Of the Year          Amount        Of the Year         Amount
    -----------         ---------      -----------        ---------
    <S>                 <C>            <C>                <C>
       1993 ........... $135,000           2001 ......... $135,000
       1994 ...........  135,000           2002 .........  135,000
       1995 ...........  135,000           2003 .........  135,000
       1996 ...........  135,000           2004 .........  135,000
       1997 ...........  135,000           2005 .........  135,000
       1998 ...........  135,000           2006 .........  135,000
       1999 ...........  135,000           2007 .........  960,000
       2000 ...........  135,000
</TABLE>
 
    The Bonds of Series W shall bear interest on the unpaid principal balance at
the rate of 6.85% per annum payable semi-annually on December 1 and June 1
commencing December 1, 1977 until due and payable and shall bear interest at the
rate of 6% per annum on any overdue principal and on any overdue installment of
interest (to the extent that payment of such interest is enforceable under
applicable
<PAGE>
 
                                       17

law). June 1, 1977 shall be the date of commencement of the first interest
period for such Bonds. All such bonds shall be dated as provided in Section 2.05
of the Original Indenture.
 
    The single Bond of Series W shall be lettered W and numbered 1 and shall,
upon issuance, be delivered by the Company to and registered in the name of the
County Trustee and shall be transferable only as required to effect an
assignment thereof to a successor County Trustee. Bonds issued upon transfer
shall be lettered W and numbered from 2 upwards and issued in the same
$2,850,000 denomination but all payments of principal theretofore made on the
Bonds of Series W shall be duly noted thereon by the Trustee.
 
    It is expected that the Company, pursuant to the Weston Agreement, will
furnish directly to the County Trustee at its principal corporate trust office
all funds required for any and all payments of principal of, and interest and
premium on, the Weston Series A Bonds (or that in lieu of any such payment the
Company or Weston will deposit with the County Trustee Weston Series A Bonds for
cancellation) and that corresponding payments of interest and of installments of
principal (including premiums if appropriate) on the Bonds of Series W will
automatically be effected in accordance with the provisions of the Weston
Agreement. Whenever payment or provisions therefor has been made in respect of
the principal, premium, if any, or interest on all or any portion of the Weston
Series A Bonds in accordance with the Weston Pollution Control Indenture, the
corresponding principal installment, premium, if any, or interest on the Bonds
of Series W shall be deemed paid to the extent such payment or provision
therefor has been made. Unless payment then is or has been made as above
provided, payment of the principal of, and premium, if any, and interest on the
single Bond of Series W shall be made to the County Trustee for the account of
Weston in funds immediately available at said office of the County Trustee, in
each case on or prior to the due date for such payment. The Trustee may at any
and all times conclusively assume that the obligation of the Company to make
payments with respect to the principal of and premium, if any, and interest on
the Bonds of Series W, so far as such payments at the time have become due, has
been fully satisfied and discharged unless and until the Trustee shall have
received a written notice from the
<PAGE>
 
                                       18

County Trustee signed by one of its officers stating (i) that timely payment of
principal of, or premium or interest on, Bonds of Series W has not been made,
(ii) that the Company is in arrears as to the payments required to be made by it
to the County Trustee pursuant to the Weston Agreement after giving effect to
any available moneys in the Bond Fund provided by the Weston Pollution Control
Indenture and (iii) the amount of the arrearage.
 
    The County Trustee, by acceptance of the single Bond of Series W, shall
agree to make prompt notation thereon of all payments and prepayments on account
of principal thereof made or occurring under any provisions of the Weston
Agreement or of this Supplemental Indenture, and to surrender said Bond to the
Trustee upon final payment thereof. If and to the extent payment of the Weston
Series A Bonds is deemed to have been made pursuant to the terms of the Weston
Pollution Control Indenture, payment of the Bonds of Series W shall be deemed to
have been made to the same extent.
 
    The Trustee is hereby appointed Registrar in respect of the Bonds of Series
W, and the principal corporate trust office of the Trustee in the Borough of
Manhattan, The City of New York, is hereby designated as the office or agency of
the Company in said Borough where notices or demands in respect of Bonds of
Series W may be served.
 
    SECTION 2.03. The text of the Bonds of Series W, and the certificate of
authentication of the Trustee to be executed thereon, are to be substantially in
the following forms, respectively:
 
No. W ....................                                            $2,850,000
 
                     BLACKS HILLS POWER AND LIGHT COMPANY
 
             FIRST MORTGAGE BOND SERIES W, 6.85%, DUE JUNE 1, 2007
 
    BLACK HILLS POWER AND LIGHT COMPANY (hereinafter called the "Company"), a
corporation organized and existing under the laws of the State of South Dakota,
for value received, hereby promises to pay to                    as trustee
or its successor
<PAGE>
 
                                       19

in trust (the "County Trustee"), under an Indenture of Trust dated as of June
1, 1977 (the "Weston Pollution Control Indenture") between Weston County,
Wyoming and said County Trustee, Two MILLION EIGHT HUNDRED FIFTY THOUSAND
DOLLARS, ($2,850,000) payable in installments as follows:
 
<TABLE>
<CAPTION>
      June 1           Principal         June 1          Principal
    Of the Year          Amount        Of the Year         Amount
    -----------         ---------      -----------        ---------
    <S>                 <C>            <C>                <C>
       1993 ........... $135,000           2001 ......... $135,000
       1994 ...........  135,000           2002 .........  135,000
       1995 ...........  135,000           2003 .........  135,000
       1996 ...........  135,000           2004 .........  135,000
       1997 ...........  135,000           2005 .........  135,000
       1998 ...........  135,000           2006 .........  135,000
       1999 ...........  135,000           2007 .........  960,000
       2000 ...........  135,000
</TABLE>
  
together with interest thereon from June 1, 1977 on the unpaid principal amount
of this Bond at the rate of 6.85% per annum until due and payable, such interest
to be paid semi-annually in arrears on December 1 and June 1 in each year.
Overdue principal and overdue installments of interest shall bear interest at
the rate of 6% per annum (to the extent that payment of such interest is
enforceable under applicable law).
  
    This Bond is one of an authorized issue of Bonds of the Company known as its
"First Mortgage Bonds", issued and to be issued in one or more series under,
and all equally and ratably secured (except as any sinking, amortization,
improvement, renewal or other analogous fund, established in accordance with
the provisions of the Indenture hereinafter mentioned, may afford additional
security for the Bonds of any particular series) by, an Indenture of Mortgage
and Deed of Trust dated as of September 1, 1941 as supplemented and amended by
Supplemental Indentures dated as of July 15, 1945, January 15, 1948, January 15,
1949, March 1, 1950, March 1, 1952, July 1, 1956, May 1, 1957, May 1, 1959,
April 1, 1960, August 1, 1960, June 1, 1961, October 1, 1962, May 1, 1963, June
1, 1969, June l5, 1974, August 1, 1974, July 15, 1975, May 1, 1976, February 15,
1977 and April 1, 1977 and subordina-
<PAGE>
 
                                       20

tion agreements dated December 29, 1950 and September 19, 1955 (hereinafter
collectively called the "Indenture"), executed by the Company to Manufacturers
Hanover Trust Company (herein called the "Trustee"), as Trustee, to which
Indenture and all indentures supplemental thereto reference is hereby made for a
description of the properties mortgaged and pledged, the nature and extent of
the security, the rights of the holders of said Bonds and the coupons
appurtenant to coupon Bonds and of the Trustee and of the Company in respect of
such security, and the terms and conditions upon which said Bonds are and are to
be issued and secured.
 
    To the extent permitted by the Indenture and as provided therein, with the
consent of the Company and upon the written consent or affirmative vote of at
least sixty-six and two-thirds per cent. in principal amount of the Bonds then
outstanding and entitled to consent, and of not less than sixty-six and
two-thirds per cent. in principal amount of the Bonds then outstanding and
entitled to consent of each series affected thereby in case one or more but less
than all of the series of Bonds issued under the Indenture are so affected, the
rights and obligations of the Company and of the holders of Bonds and coupons
appurtenant to coupon Bonds, and the terms and provisions of the Indenture and
of any instrument supplemental thereto may be modified from time to time,
provided that no such modification or alteration shall be made which would
postpone the date fixed herein or in the coupons or in the Indenture for the
payment of the principal of, or any installment of interest on, the Bond, or
reduce the principal of, or the rate of interest payable on, the Bonds, or
reduce the percentage of the principal amount of Bonds the consent of which is
required for the authorization of any such modification or alteration, or which
would modify, without the written consent of the Trustee, the rights, duties or
immunities of the Trustee.
 
    As provided in said Indenture, said Bonds are issuable in series which may
vary as in said Indenture provided or permitted. This Bond is one of a series of
Bonds entitled "First Mortgage Bonds Series W".
 
    Weston County, Wyoming (the "County") is issuing and selling, or has
issued and sold, its Pollution Revenue Bonds (Black
<PAGE>
 
                                       21

Hills Power and Light Company Project) Collateralized 1977 Series A (hereinafter
called the "Weston Series A Bonds") under and pursuant to the Weston Pollution
Control Indenture. This Bond of Series W is issued as a means of securing
repayment of the loan of the proceeds of the Weston Series A Bonds to the
Company to finance the acquisition, construction and improvement of certain
pollution control facilities pursuant to a Financing Agreement dated as of June
1, 1977 (the "Weston Agreement"). It is expected that the Company will make
all payments of principal of, and interest and premium on, the Weston Series A
Bonds directly to the County Trustee (or that in lieu thereof the Company or
Weston will deposit with the County Trustee Weston Series A Bonds for
cancellation). All such payments shall automatically constitute corresponding
payments on the Bonds of Series W in accordance with the provisions of the
Weston Agreement. The holder of this Bond by acceptance hereof agrees that
whenever any payment or prepayment on account of the principal of this Bond is
made or occurs under any provision of the Indenture, the Weston Agreement or the
Weston Pollution Control Indenture, the holder hereof shall promptly note on the
Schedule of Payments of Principal, the Schedule of Prepayments of Principal or
the Schedule of Credits of Prepayments of Principal the date and amount of each
such payment or prepayment of principal, and shall promptly notify the Trustee
of the amount of each such payment and that the notation of payment has been
duly made, and further agrees to surrender this Bond to the Trustee for
cancellation when all principal of, premium, if any, and interest on this Bond
shall have been duly paid. The holder of this Bond further agrees that if
payment of the Weston Series A Bonds is deemed to have been made pursuant to the
provisions of the Weston Pollution Control Indenture, payment of this Bond shall
be deemed to have been made.
 
    Unless payment then is or has been made pursuant to the foregoing paragraph,
the principal of and premium, if any, and interest on this Bond will be paid in
lawful money of the United States of America and will be payable at the
principal corporate trust office of the County Trustee to the County Trustee for
the account of Weston in immediately available funds at said office of the
County Trustee, in each
<PAGE>
 
                                       22

case on or prior to the due date for such payment. The holder of this Bond by
acceptance hereof agrees that any such payment of principal, premium or interest
on this Bond shall be credited as and used to make a corresponding payment of
principal, premium or interest on the Weston Series A Bonds.
 
    This Bond is not prepayable prior to June 1, 1987, except that if at any
time (1) the Company shall elect to prepay installments payable under the Weston
Agreement and to cause the Weston Series A Bonds to be redeemed upon the
occurrence of an event specified in Section 8.1 of the Weston Agreement or (2)
the Company shall be obligated to prepay installments payable under the Weston
Agreement and to cause the Weston Series A Bonds to be redeemed upon the
occurrence of an event specified in Section 8.2 of the Weston Agreement, in any
such event this Bond shall be prepaid by the Company in whole, but not in part
(except that upon the occurrence of an event specified in paragraph (c) of
Section 8.2 of the Weston Agreement the Bond shall be prepaid in whole or in
part) at 100%, of the unpaid principal amount thereof plus accrued interest to
the prepayment date, which date shall be the same date as the prepayment date
under the Weston Agreement determined as set forth in said Sections 8.1 and 8.2.
The principal amount of Bonds of Series W to be prepaid at any time upon the
occurrence of the event specified in paragraph (c) of Section 8.2 of the Weston
Agreement shall be the same as the principal amount of Weston Series A Bonds to
be redeemed as the result of the occurrence of such event.
 
    This Bond shall be prepaid by the Company on or after June 1, 1987, in whole
at any time or in part on any interest payment date, if but only if the Company
shall have elected to prepay installments under the Weston Agreement of like
principal amount and to cause Weston Series A Bonds of like principal amount to
be redeemed on the prepayment date in accordance with the second paragraph of
Section 3.01 of the Weston Pollution Control Indenture. Such prepayment shall be
at the prepayment price determined in accordance with the following table plus
accrued interest to the prepayment date:
<PAGE>
 
                                       23

<TABLE>
<CAPTION>
 If prepayment date is during
twelve-month period beginning                         Prepayment
           June l                                        Price
- -----------------------------                         ----------
<S>                                                   <C>
            1987 ..................................... 103    %
            1988 ..................................... 102 1/2
            1989 ..................................... 102
            1990 ..................................... 101 1/2
            1991 ..................................... 101
            1992 ..................................... 100 1/2
            1993 and thereafter ...................... 100
</TABLE>

    Except to the extent that the Company shall at any time elect, pursuant to
the provisions of the Weston Pollution Control Indenture, to apply as a credit
in respect of a sinking fund obligation thereunder an amount of Weston Series A
Bonds redeemed or purchased and delivered to the County Trustee, any prepayment
of only a part of this Bond shall be in inverse order of the maturities of the
several installment payments, e.g., all the portion of this Bond due on June 1,
2007, must be prepaid before any installment having an earlier maturity date. In
the event that the Company at any time elects, pursuant to the provisions of the
Weston Pollution Control Indenture, to apply as a credit in respect of a sinking
fund obligation thereunder an amount of Weston Series A Bonds redeemed or
purchased and delivered to the County Trustee, the corresponding prepayment of
this Bond in the same principal amount shall be credited against the installment
due on this Bond in the same year as such sinking fund obligation.

    In each case where this Bond is to be prepaid in whole or in part as
contemplated herein notice of not less than thirty (30) nor more than sixty (60)
days shall be given by first class mail postage prepaid to the holder of record
of this Bond unless such notice has been waived in writing by the County
Trustee.

    If an event of default as defined in the Indenture shall occur, the
principal of this Bond may become and be declared due and payable in the manner
and with the effect provided in the Indenture. No holder of this Bond
shall have any right to institute any suit or proceeding for the foreclosure of
the Indenture or for any other remedy thereunder, except to the extent and in
the manner set forth in the Indenture.
<PAGE>
 
                                       24

    This Bond shall be non-transferable except as required to effect (i) a
transfer to any successor trustee under the Weston Pollution Control Indenture
or (ii) a transfer after an event of default under the Weston Pollution Control
Indenture in the course of the exercise of rights and remedies consequent upon
such event of default. Subject to such restrictions, this Bond is transferable
by the registered holder hereof in person or by attorney duly authorized in
writing, at the office of Manufacturers Hanover Trust Company, New York, New
York, on registry books kept for such purpose at such office. No charge will be
made by the Company for any such transfer of this Bond. Before any transfer of
this Bond by the County Trustee or attorney duly authorized will be recognized
or given effect by the Company or the Trustee, the County Trustee shall note
hereon the date to which interest has been paid as well as the amounts of all
principal payments and prepayments hereon, and shall notify the Company and the
Trustee of the name and address of the transferee and shall afford the Company
and the Trustee the opportunity of verifying the notation as to payment of
interest and principal.

    No recourse shall be had for the payment of the principal of or the interest
on this Bond or for any claim based hereon or otherwise in respect hereof or of
the Indenture or of any agreement supplemental thereto against any subscriber to
the capital stock, incorporator or any past, present or future stockholder,
officer or director of the Company or of any predecessor or successor
corporation, either directly or through the Company or any predecessor or
successor corporation or any receiver or trustee in bankruptcy, whether by
virtue of any constitution or statute or rule of law or by the enforcement of
any assessment or penalty or stock subscription or otherwise, all such
liability, whether at common law or in equity or by statute or constitution or
otherwise, being, to the extent permitted by law, by the acceptance and as a
part of the consideration for the issuance hereof, expressly waived and released
by the holder hereof.

    This Bond shall not be valid or become obligatory for any purpose until the
certificate of authentication hereon shall have been signed by the Trustee, or
its successor as Trustee, under said Indenture.
<PAGE>
 
                                       25

    In WITNESS WHEREOF, the Company has caused this Bond to be signed in its
name by its President or one of its Vice-Presidents, and its corporate seal to
be impressed or imprinted hereon and attested by its Secretary or one of its
Assistant Secretaries.
 
    Dated,
 
                                            BLACK HILLS POWER AND LIGHT COMPANY,
 
                                               By ..............................
                                                                     President
 
Attest:
 
    ...................................
                            Secretary
 
 
                        (FORM OF TRUSTEE'S CERTIFICATE)

    This is one of the Bonds, of the series designated therein, described in the
within mentioned Indenture.
 
                                            MANUFACTURERS HANOVER TRUST COMPANY,
                                                                     as Trustee,
 
                                               By ..............................
                                                           Authorized Officer.
<PAGE>
 
                                       26

                      SCHEDULE OF PAYMENTS OF PRINCIPAL
<TABLE>
<CAPTION>
 
 Amount of          Date             Date of             Authorized Official
Installment         Due              Payment                  and Title
- -----------         ----             -------             -------------------
<S>                 <C>              <C>                 <C>
</TABLE>
 
 
 
 
 
                     SCHEDULE OF PREPAYMENTS OF PRINCIPAL
<TABLE>
<CAPTION>
 
        Principal Amount                        Authorized Official
             Prepaid         Date Prepaid            and Title
        ----------------     ------------       -------------------
        <S>                  <C>                <C>
</TABLE>
 
 
 
 
 
               SCHEDULE OF CREDITS OF PREPAYMENTS OF PRINCIPAL*
<TABLE>
<CAPTION>
 
                      Date of Installment
Prepaid Principal           to which                         Authorized Official
 Amount Credited            Credited        Date Credited         and Title
- -----------------     -------------------   -------------    -------------------
<S>                   <C>                   <C>              <C>
</TABLE>
 
 
 
    *This Schedule to be completed only in the event that a prepayment is
credited against an installment maturing prior to the then last maturing unpaid
installment. Each prepayment is credited against the then last maturing unpaid
installment unless Weston Series A Bonds redeemed or purchased and delivered to
the County Trustee are credited against a sinking fund obligation under the
Weston Pollution Control Indenture, whereupon a corresponding prepayment in the
same principal amount shall be credited against the installment due in the same
year as such sinking fund obligation and entered in this Schedule.

    (NOTICE: The within Bond may not be transferred until this schedule has been
verified by the Trustee.)
<PAGE>
 
                                       27

    SECTION 2.04. The single Bond of Series W is not prepayable prior to June
1, 1987, except that if, at any time, (1) the Company shall elect to prepay
installments payable under the Weston Agreement and to cause the Weston Series A
Bonds to be redeemed upon the occurrence of an event specified in Section 8.1 of
the Weston Agreement or (2) the Company shall be obligated to prepay
installments payable under the Weston Agreement and to cause the Weston Series A
Bonds to be redeemed upon the occurrence of an event specified in Section 8.2 of
the Weston Agreement, in any such event the Bonds of Series W shall be prepaid
by the Company in whole, but not in part (except that upon the occurrence of an
event specified in paragraph (c) of Section 8.2 of the Weston Agreement the
Bonds of Series W shall be prepaid in whole or in part) at 100% of the unpaid
principal amount thereof plus accrued interest to the prepayment date, which
date shall be the same date as the prepayment date under the Weston Agreement
determined as set forth in said Sections 8.1 and 8.2. The principal amount of
Bonds of Series W to be prepaid at any time upon the occurrence of the event
specified in paragraph (c) of Section 8.2 of the Weston Agreement shall be the
same as the principal amount of Weston Series A Bonds to be redeemed as the
result of the occurrence of such event.
 
    The single Bond of Series W shall be prepaid by the Company on or after June
1, 1987, in whole at any time or in part on any interest payment date, if but
only if the Company shall have elected to prepay installments under the Weston
Agreement of like principal amount and to cause Weston Series A Bonds of like
principal amount to be redeemed on the prepayment date in accordance with the
second paragraph of Section 3.01 of the Weston Pollution Control Indenture. Such
prepayment shall be at the prepayment price determined in accordance with the
following table plus accrued interest to the prepayment date:

<TABLE>
<CAPTION>
If the prepayment date is
 during the twelve months                                Prepayment
     beginning June 1                                       Price
- -------------------------                                ----------
<S>                                                      <C>
           1987 ......................................... 103    %
           1988 ......................................... 102 1/2
           1989 ......................................... 102
           1990 ......................................... 101 1/2
           1991 ......................................... 101
           1992 ......................................... 100 1/2
           1993 and thereafter........................... 100
</TABLE>
<PAGE>
 
                                       28

    Except to the extent that the Company shall at any time elect, pursuant to
the provisions of the Weston Pollution Control Indenture, to apply as a credit
in respect of a sinking fund obligation thereunder an amount of Weston Series A
Bonds redeemed or purchased and delivered to the County Trustee, any prepayment
of only a part of the single Bond of Series W shall be in inverse order of the
maturities of the several installment payments, e.g., all the portion of the
single Bond of Series W due on June 1, 2007, must be prepaid before any
installment having an earlier maturity date. In the event that the Company at
any time elects, pursuant to the provisions of the Weston Pollution Control
Indenture, to apply as a credit in respect of a sinking fund obligation
thereunder an amount of Weston Series A Bonds redeemed or purchased and
delivered to the County Trustee, the corresponding prepayment of the single Bond
of Series W in the same principal amount shall be credited against the
installment due on the single Bond of Series W in the same year as such sinking
fund obligation.

    If Bonds of Series W are to be redeemed in whole or in part as provided in
this Section 2.04, notice of redemption shall be given by first class mail,
postage prepaid, by or on behalf of the Company, not less than thirty (30) nor
more than sixty (60) days prior to the date of redemption, to the registered
holders of all Bonds so to be redeemed, at their respective addresses appearing
upon the books maintained by the Trustee pursuant to Section 2.10 of the
Original Indenture. Any notice which is mailed as herein provided shall be
conclusively presumed to have been properly and sufficiently given on the date
of such mailing, whether or not the registered holder or payee, as the case may
be, receives the notice. In case of any redemption of Bonds of such Series by
the Trustee pursuant to the provisions of the Indenture or any indenture
supplemental thereto, notice of redemption shall be given in a similar manner by
the Trustee. Notwithstanding any provision of Article Ten of the Original
Indenture, no publication of notice of redemption of any Bonds of Series W shall
be required by the Indenture.

    Except as provided in the immediately preceding paragraph, the provisions of
Article Ten of the Original Indenture shall in all respects apply to any
redemption to which this Section 2.04 applies. Nothing in this Section 2.04
contained shall affect the manner of giving notice of
<PAGE>
 
                                       29

the redemption of Bonds of the Company of any series other than Bonds of Series
W.
 
    Notwithstanding the provisions of the Original Indenture (including
particularly Section 8.08 (b) thereof), the single Bond of Series W is subject
to redemption prior to maturity only upon the events and as set forth in this
Section 2.04.
 
                                 ARTICLE THREE
 
                           MISCELLANEOUS PROVISIONS
 
    SECTION 3.01. The Company, by the execution hereof, acknowledges that a true
copy of this Supplemental Indenture has been delivered to and received by it.

    SECTION 3.02. Except as heretofore supplemented and amended and as amended
by this Supplemental Indenture, all the provisions, terms and conditions of the
Original Indenture shall continue in full force and effect.

    SECTION 3.03. This Supplemental Indenture may be executed in several
counterparts, all or any of which may be treated for all purposes as one
original and shall constitute and be one and the same instrument.

    IN WITNESS WHEREOF, BLACK HILLS POWER AND LIGHT COMPANY, party hereto of the
first part, has caused this Supplemental Indenture to be executed on its behalf
by the Chairman of its Board, its President or one of its Vice Presidents and
its corporate seal to be hereto affixed and to be attested by its Secretary or
an Assistant Secretary, and MANUFACTURERS HANOVER TRUST COMPANY, party hereto of
the second part, in evidence of its acceptance of the trust hereby created, has
caused this Supplemental Indenture to be executed on its behalf by one of its
Vice Presidents or Assistant Vice Presidents and its corporate
<PAGE>
 
                                       30

seal to be hereto affixed and to be attested by its Secretary or an Assistant
Secretary, all as of the day and year first above written.
 
                                            BLACK HILLS POWER AND LIGHT COMPANY,
 
                                                     NEIL G. SIMPSON
                                                       Chairman of the Board.
 
[CORPORATE SEAL]
 
Attest:
 
    GEORGE T. LOCKE
                     Secretary.
 
Signed, sealed and delivered by
  BLACK HILLS POWER AND LIGHT
  COMPANY in the presence of:
 
    LOUISE S. KELLEY
    BARRY D. WILSON
 
 
                                            MANUFACTURERS HANOVER TRUST COMPANY,
 
                                                      E. F. COCKINGS
                                                       Assistant Vice President.
 
[CORPORATE SEAL]
 
Attest:
 
    K. C. KELLY
           Assistant Secretary.
 
Signed, sealed and delivered by
  MANUFACTURERS HANOVER TRUST
  COMPANY in the presence of:
 
    JAMES M. FOLEY
    D. A. URSITTI, JR.
<PAGE>
 
                                       31

STATE OF SOUTH DAKOTA
                        SS.:
COUNTY OF PENNINGTON 
 
    On this 26th day of May, 1977, before me, MARY S. KELLER, the undersigned
officer, personally appeared NEIL G. SIMPSON, to me personally known, who
acknowledged himself to be, and being by me duly sworn, did say that he is
Chairman of the Board of BLACK HILLS POWER AND LIGHT COMPANY, a corporation, and
that the seal affixed to the foregoing instrument is the corporate seal of said
corporation and that said instrument was executed by, and signed in the name of,
the corporation, by him, as such Chairman of the Board and sealed in behalf of
the corporation by authority of its Board of Directors for the purposes therein
contained, and the said NEIL G. SIMPSON acknowledged the same as the free act
and deed of said corporation.
 
    IN WITNESS WHEREOF, I hereunto set my hand and official seal.
 
                                            MARY S. KELLER
 
                                             Notary Public
 
[NOTARIAL SEAL]
 
 
 
My commission expires June 27, 1979
<PAGE>
 
                                       32

STATE OF NEW YORK
                      SS.:
COUNTY OF NEW YORK
 
On this 7th day of June, 1977, before me, MARY LEONARDI, the undersigned
officer, personally appeared E. F. COCKINGS, to me personally known, who
acknowledged himself to be, and being by me duly sworn, did say that he is
Assistant Vice President of MANUFACTURERS HANOVER TRUST COMPANY, a corporation,
and that the seal affixed to the foregoing instrument is the corporate seal of
said corporation and that said instrument was executed by, and signed in the
name of, the corporation, by him, as such Assistant Vice President and sealed in
behalf of the corporation by authority of its Board of Directors for the
purposes therein contained, and the said E. F. COCKINGS acknowledged the same as
the free act and deed of said corporation.
 
    IN WITNESS WHEREOF, I hereunto set my hand and official seal.
 
 
                                               MARY LEONARDI
                                               Notary Public
 
[NOTARIAL SEAL]                                MARY LEONARDI
                                      Notary Public, State of New York
                                               No. 40-7495520
                                         Qualified in Putnam County
                                    Certificate filed in New York County
                                     Commission Expires March 30, 1978

<PAGE>
 
                                                   Conformed Copy
 
================================================================================

                           BLACK HILLS CORPORATION

                                      TO

                     MANUFACTURERS HANOVER TRUST COMPANY,


                                                As Trustee

                                  ----------

                                 TWENTY THIRD
                            SUPPLEMENTAL INDENTURE


                        Dated as of September 1, 1986

                                  ----------

                  Supplemental to Indenture of Mortgage and
                  Deed of Trust Dated as of September 1, 1941


          First Mortgage Bonds, Series X, 8.375%, Due October 1, 1998
 
================================================================================

<PAGE>
 
         TWENTY THIRD SUPPLEMENTAL INDENTURE, dated as of the 1st day of
September, 1986, between Black Hills Corporation (which was formerly named
Black Hills Power and Light Company and which now operates its electric utility
division under the assumed name of Black Hills Power and Light Company), a
corporation organized and existing under the laws of the State of South Dakota
(hereinafter called the "Company"), party of the first part, and Manufacturers
Hanover Trust Company, a corporation organized and existing under the laws of
the State of New York, as Trustee under the Indenture hereinafter mentioned
(hereinafter called the "Trustee"), party of the second part.

         WHEREAS, in order to secure an authorized issue of First Mortgage
Bonds of the Company, the Company has executed and delivered an Indenture of
Mortgage and Deed of Trust to Central Hanover Bank and Trust Company
(subsequently known as The Hanover Bank), as Trustee, dated September 1, 1941,
hereinafter referred to as the "Original Indenture", and has also executed and
delivered to said Trustee and to Manufacturers Hanover Trust Company (which on
September 8, 1961 became the Trustee under the Original Indenture, as
theretofore supplemented and amended, by virtue of the merger of said The
Hanover Bank into Manufacturers Trust Company, under said name Manufacturers
Hanover Trust Company), as Trustee, various Supplemental Indentures
supplementing and/or modifying the Original Indenture, respectively dated as
follows:
 
<TABLE>
          <S>                                        <C>
          First Supplemental Indenture ..............July 15, 1945
          Second Supplemental Indenture .............January 15, 1948
          Third Supplemental Indenture ..............January 15, 1949
          Fourth Supplemental Indenture .............March 1, 1950
          Fifth Supplemental Indenture ..............March 1, 1952
          Sixth Supplemental Indenture ..............July 1, 1956
          Seventh Supplemental Indenture   ..........May 1, 1957
          Eighth Supplemental Indenture .............May 1, 1959
          Ninth Supplemental Indenture ..............April 1, 1960
          Tenth Supplemental Indenture ..............August 1, 1960
          Eleventh Supplemental Indenture ...........June 1, 1961
          Twelfth Supplemental Indenture ............October 1, 1962
          Thirteenth Supplemental Indenture .........May 1, 1963
          Fourteenth Supplemental Indenture .........June 1, 1969
</TABLE>
 
<PAGE>
 
                                       2

<TABLE>
          <S>                                       <C>
           Fifteenth Supplemental Indenture ........June 15, 1974
           Sixteenth Supplemental Indenture ........August 1, 1974
           Seventeenth Supplemental Indenture ......July 15, 1975
           Eighteenth Supplemental Indenture .......May 1, 1976
           Nineteenth Supplemental Indenture .......February 15, 1977
           Twentieth Supplemental Indenture ........April 1, 1977
           Twenty First Supplemental Indenture .....June 1, 1977
           Twenty Second Supplemental Indenture ....July 14, 1982

</TABLE>

and the Company also executed and delivered to said The Hanover Bank, as
Trustee, Subordination Agreements dated December 29, 1950 and September 19,
1955 also supplementing the Original Indenture, which, as supplemented and
amended by said twenty-two Supplemental Indentures and said Subordination
Agreements, is hereinafter referred to as the "Indenture"; and

         WHEREAS, pursuant to the provisions of the Indenture, First Mortgage
Bonds have been duly issued and are presently outstanding and secured by the
Indenture as follows:

<TABLE>
<CAPTION>

                                                       Principal Amount
                               Series                     Outstanding
                               ------                  ----------------
           <S>                                            <C>
           Series H, 5-1/4%, due May 15, 1987....         $   638,000
           Series I, 5-1/8%, due May 1, 1989.....             870,000
           Series J, 5-3/4%, due April 1, 1990...             875,000
           Series K, 5-5/8%, due August 1, 1990..             875,000
           Series L, 5-3/8%, due June 1, 1991....             880,000
           Series M, 5%, due October 1, 1992.....             880,000
           Series N, 4-3/4%, due May 1, 1993.....             889,000
           Series O, 8.05%, due June 1, 1999.....           4,915,000
           Series P, 10.75%, due August 1, 2004..           5,790,000
           Series R, 6-5/8%, due April 1, 2007...           5,8S0,000
           Series S, 6-5/8%, due April 1, 2007...           2,050,000
           Series T, 6-5/8%, due April 1, 2007...           1,000,000
           Series U, 6-5/8%, due April 1, 2007...           1,000,000
           Series V, 6.85%, due June 1, 2007.....           1,550,000
           Series W, 6.85%, due June 1, 2007.....           2,850,000
                                                          -----------
                                                          $30,972,000
                                                          ===========
</TABLE>
<PAGE>
 
                                       3




; and

         WHEREAS, the Company has acquired certain property not heretofore
specifically described under the Indenture and it desires, to specifically
describe such property; and

         WHEREAS, as permitted by the Indenture the Company by resolutions of
its Board of Directors duly adopted, has determined to create a new series of
bonds to be known as its "First Mortgage Bonds, Series X, 8.375%, due October
1, 1998" (herein called "Bonds of Series X"), to be initially authenticated and
delivered in the aggregate principal amount of $6,000,000, in the form, having
the characteristics and being entitled to the benefits as in this Supplemental
Indenture provided; and

         WHEREAS, the Company, in the exercise of the powers and authority
conferred upon and reserved to it under and by virtue of the provisions of the
Indenture, and particularly the provisions contained in Articles Two and
Seventeen thereof, and pursuant to appropriate resolutions of its Board of
Directors, has duly resolved and determined to make, execute and deliver to the
Trustee a Twenty Third Supplemental Indenture in the form hereof (herein
sometimes referred to as "this Supplemental Indenture") for the purposes herein
provided; and

         WHEREAS, all conditions and requirements necessary to make this
Supplemental Indenture a valid, binding and legal instrument in accordance with
its terms have been done, performed and fulfilled, and the execution and
delivery hereof have been in all respects duly authorized;

        NOW, THEREFORE, THIS INDENTURE WITNESSETH: That the Company, in
consideration of the premises and of one dollar to it duly paid by the Trustee
at or before the ensealing and delivery of these presents, the receipt whereof
is hereby acknowledged, and of other good and valuable consideration, in order
to better secure the payment both of the principal of and interest on all Bonds
issued under the Indenture and that may be issued under this or any other
indentures supplemental thereto, according to their tenor and effect, and the
performance by the Company of all the covenants and conditions herein and
therein contained, and in order to establish the terms of the Bonds of Series
X, hereby further covenants and agrees to and with the Trustee and its
successors in the trust under the Indenture for the benefit of all those who
shall from time to time hold the Bonds of Series X as follows:
<PAGE>
 
                                       4







          The Company does hereby ratify and confirm its Mortgage and Pledge to
the Trustee of all property described in the Indenture and does hereby by these
presents grant, bargain, sell, warrant, alien, remise, release, convey,
confirm, assign, mortgage, pledge and set over unto the Trustee, and to its
successors and assigns forever, the following described property acquired by
the Company and not heretofore specifically described under the Indenture:

                       LANDS IN WESTON COUNTY, WYOMING


Tract 1

     The Northwest Quarter of Section 14, Township 46 North, Range 63 West,
     containing 160 acres, more or less.

Tract 2

     The West Half of the Southwest Quarter of Section 14, North Half of the
     Southeast Quarter of Section 15 and all land east of railroad in the
     Northeast Quarter of the Southwest Quarter of Section 15, Township 46
     North, Range 63 West, containing 180 acres, more or less.

                   LANDS IN PENNINGTON COUNTY, SOUTH DAKOTA

Tract 1

     North 200 feet (Lot 1) of Tract D in West Half of the Southwest Quarter of
     Section 17, Township 1 North, Range 8 East, Black Hills Meridian.

Tract 2

     That portion of Lot 23 of Section 30, Township 15 North, Range 5 East,
     Black Hills Meridian, lying south of the ROW in tract A of Reder Placer.

Tract 3

     Tract One of the Krebs Subdivision in the Northwest Quarter of the
     Southeast Quarter of Section 34 and Lots F, G and H in the Southeast
     Quarter of the Southwest Quarter of the Northeast Quarter of Section 34,
     Township 2 North, Range 7 East, Black Hills Meridian, containing 10.169
     acres, more or less.
<PAGE>
 
                                       5


Tract 4

     The South 180 feet of Lot 3 of the BHP addition in the Northwest Quarter of
     the Southeast Quarter of Section 34, Township 2 North, Range 7 East, Black
     Hills Meridian, in the City of Rapid City.

Tract 5

     Lots 1 and 2 of the BHP addition in the City of Rapid City.

                   LAND IN FALL RIVER COUNTY, SOUTH DAKOTA

     Lots 11 and 12 of Block 1 of the Original Town of Edgemont.

                      LAND IN BUTTE COUNTY, SOUTH DAKOTA

     Lot 13 of Block 18 of the Original Town of Belle Fourche.

                      LAND IN MEADE COUNTY, SOUTH DAKOTA

     Lot 17 of Block 14 of the Original Town of Sturgis.

                    LANDS IN LAWRENCE COUNTY, SOUTH DAKOTA
Tract 1


     A portion of Mineral Surveys 400 and 1521. Beginning at Corner No. 1 of the
     Springville Lode, M.S.1521, identical with Corner No. 3 of the Robert
     Calvin Lode, M.S.1521, for Corner No. 1 of said Tract, thence running South
     14 degrees 39' East to Corner No. 2 about 60 feet; thence North 75 degrees
     21' East about 150 feet to Corner No. 3; thence North 14 degrees 39' West
     about 110 feet to Corner No. 4; thence South 75 degrees 21' West about
     156.2 feet to Corner No. 5 on Line 2-3 of the Robert Calvin Lode; thence
     South 21 degrees 45' East about 50.4 feet to the place of beginning.

Tract 2

     Tracts A, B and C of Subdivision of Mineral Surveys 400, 1050, 1305, 1521
     and 1522 in Section 4, Township 4 North, Range 3 East, Black Hills
     Meridian.
<PAGE>
 
                                       6


          TOGETHER with all contracts, agreements, rights and understandings,
whether now owned or hereafter acquired for the purchase or exchange of
electric energy or for the making of connections for exchange of energy or
service, including, but not limited to:

 Power Sales Agreement Between Pacific Power & Light Company and Black Hills
 Power and Light Company, dated as of December 31, 1983 providing for the
 purchase by the Company of electric power and energy for a term commencing
 January 1, 1984 and terminating December 31, 2023;

          TOGETHER with, all and singular, the tenements, hereditaments and
appurtenances belonging or in any wise appertaining to said property or any part
thereof, with the reversion and reversions, remainder and remainders, tolls,
rents, revenues, issues, earnings, income, products and profits thereof, and
all the estate, right, title and interest and claim whatsoever, at law as well
as in equity, which the Company now has or may hereafter acquire in and to said
property, and every part and parcel thereof;

          TOGETHER with all buildings, structures and improvements erected or
constructed on any of the above described tracts of land;

          TOGETHER with all property real, personal and mixed situated upon,
connected with, used in connection with, or in any wise appertaining to any of
the property described in the Indenture or in this Supplemental Indenture,
presently owned by the Company (other than Excepted Property as defined in the
Indenture), and including all rights of way, easements, licenses, permits, and
privileges in connection with or pertaining to all of such property, whether
existing by reason of express or implied, written or oral, evidences of
ownership, rights, agreements or understandings;

          TOGETHER with all electric generating plants and electric
transmission and distribution systems now owned by the Company, and any
additions to or extensions of any such electric generating plants and electric
transmission and distribution systems, together with all engines, dynamos,
motors, generators, boilers, turbines, pole lines, poles, wires, cross-arms,
insulators, transformers, meters, buildings, erections, structures, stations,
substations, powerhouses, power producing and power transmitting equipment,
water, water rights, waterwheels, headworks, race-ways, hydraulic works, hydro-
electric plants, cables, conduits, tools, instruments, apparatus, appliances,
machinery, facilities, fixtures and all other property used or
<PAGE>
 
                                       7


provided for use in the construction, repair, maintenance or operation thereof
(other than Excepted Property as defined in the Indenture), and together also
with all the rights, privileges, franchises, easements, licenses, ordinances,
rights of way, liberties, immunities and permits of the Company howsoever
conferred, or acquired with respect to the construction, maintenance, repair or
operation of said electric generating plants and electric transmission or
distribution systems, and each of them, and any additions thereto and extension
thereof (EXCEPT, and not including the Project and Project Site, and all
improvements and appurtenances thereto as defined in and all of which are
released from the lien of the Indenture by the provisions of Article One of the
Eighteenth Supplemental Indenture, dated as of May 1, 1976);

        TO HAVE AND TO HOLD all such properties, real, personal and mixed,
mortgaged, pledged or conveyed by the Company as aforesaid, or intended so to
be, unto the Trustee and its successors and assigns forever;

        IN TRUST, NEVERTHELESS, for the same purposes and upon the same trusts,
terms and conditions, and subject to and with the same provisions and covenants
as are set forth in the Indenture (as amended by this Supplemental Indenture)
with the same effect in all respects as if the property and rights herein
described and herein conveyed to the Trustee had, at the time of the execution
and delivery of the Original Indenture, been owned by the Company and had been
specifically and at length described in and conveyed to the Trustee by the
Original Indenture as a part of the property therein stated to be conveyed.

                                 ARTICLE ONE
                          Bonds of Series X, 8.375%

        SECTION 1.01. There is hereby created a series of Bonds, known as and
entitled "First Mortgage Bonds, Series X, 8.375%, due October 1, 1998", and the
form thereof shall be as provided in this Supplemental Indenture.

        The aggregate principal amount of Bonds of Series X which may be
authenticated and delivered and outstanding under the Indenture and this
Supplemental Indenture shall be limited in aggregate principal amount to
$6,000,000, except as provided under Section 2.12 of the Indenture. The Bonds
of Series X shall bear interest at the rate of 8.375% per annum until
<PAGE>
 
                                       8


the principal thereof becomes due and payable and thereafter, if default be
made in the payment of such principal, at the rate of 9.375% per annum until
the principal thereof shall be paid, and shall mature October 1, 1998.

           The Bonds of Series X shall be registered Bonds without coupons of
the denominations of $1,000 and any multiples of $1,000 which may be executed
by the Company and delivered to the Trustee for authentication and delivery.
The date of commencement of the first interest period for the Bonds of Series X
shall be the date of initial authentication and delivery thereof. The Bonds of
Series X shall be dated as provided in Section 2.05 of the Indenture, and shall
be numbered, from XRX1 consecutively upwards. All Bonds of Series X shall bear
interest from their respective dates, such interest to be payable semi-annually
on the first day of April and October in each year. The first interest payment
shall be due October 1, 1986 and shall include interest from the date of
initial authentication and delivery thereof. The principal of, premium, if any,
and interest on the Bonds of Series X shall be payable at the principal
corporate trust office of the Trustee, in the Borough of Manhattan, The City of
New York, in any coin or currency of the United States of America which at the
time of payment shall be legal tender for the payment of public and private
debts. The Bonds of Series X shall be subject to redemption as provided in
Sections 1.03 and 1.04 and Article Two of this Supplemental Indenture.

           The Trustee is hereby appointed Registrar in respect of the Bonds of
Series X, and the principal corporate trust office of the Trustee in the
Borough of Manhattan, The City of New York, is hereby designated as the office
or agency of the Company in said Borough where notices or demands in respect of
Bonds of Series X may be served.

           The definitive Bonds of Series X shall be issued in such printed
form as approved by the Trustee. Subject to the foregoing provisions of this
Section and to the provisions of Section 2.11 of the Indenture, all definitive
Bonds of Series X shall, upon surrender thereof to the Trustee at its principal
office, be exchangeable for other Bonds of the same series, in registered form
and in such authorized denomination or denominations in the same aggregate
principal amount, as may be requested by the holder surrendering the same. The
Company will execute, and the Trustee shall authenticate and deliver,
registered Bonds of Series X without coupons, whenever the same shall be
required for any such exchange.
 
<PAGE>
 
                                       9







        SECTION 1.02. The text of the Bonds of Series X, and the certificate of
authentication of the Trustee to be executed thereon, are to be substantially
in the following forms, respectively:

                          (FORM OF BOND OF SERIES X)

No. XRX .....                                                  $..............

                           BLACK HILLS CORPORATION
          FIRST MORTGAGE BOND, SERIES X, 8.375%, DUE OCTOBER 1, 1998

        BLACK HILLS CORPORATION (hereinafter called the "Company"), a
corporation organized and existing under the laws of the State of South Dakota,
for value received, hereby promises to pay to                           , or
registered assigns, on the 1st day of October, 1998, at the principal corporate
trust office of the Trustee, in the Borough of Manhattan, The City of New York,
              Dollars, in any coin or currency of the United States of America
which at the time of payment shall be legal tender for the payment of public and
private debts, and to pay interest thereon from the date hereof, at the rate of
8.375 per cent, per annum (computed on the basis of a 360-day year of 12 thirty-
day months), payable at said principal office of the Trustee in like coin or
currency semi-annually on April 1 and October 1 in each year until the principal
hereof shall have become due and payable, and thereafter if default be made in
the payment of such principal, at the rate of 9.375 per cent, per annum until
the principal hereof shall be paid. The first interest payment shall be due
October 1, 1986 and shall include interest from the date of initial
authentication and delivery thereof.

        This Bond is one of an authorized issue of Bonds of the Company known
as its "First Mortgage Bonds", issued and to be issued in one or more series
under, and all equally and ratably secured (except as any sinking,
amortization, improvement, renewal or other analogous fund, established in
accordance with the provisions of the Indenture hereinafter mentioned, may
afford additional security for the Bonds of any particular series) by, an
Indenture of Mortgage and Deed of Trust dated as of September 1, 1941 executed
by the Company to Central Hanover Bank and Trust Company (subsequently known 
as The Hanover Bank and herein, with its successor by merger, Manufacturers
Hanover Trust Company, called the "Trustee"), as
<PAGE>
 
                                       10



Trustee, as supplemented and amended by Supplemental Indentures dated as of
July 15, 1945, January 15, 1948, January 15, 1949, March 1, 1950, March 1,
1952, July 1, 1956, May 1, 1957, May 1, 1959, April 1, 1960, August 1, 1960,
June 1, 1961, October 1, 1962, May 1, 1963, June 1, 1969, June 15, 1974, August
1, 1974, July 15, 1975, May 1, 1976, February 15, 1977, April 1, 1977, June 1,
1977, July 14, 1982 and September 1, 1986 and Subordination Agreements dated
December 29, 1950 and September 19, 1955 (said original Indenture as so
supplemented and amended being hereinafter collectively called the
"Indenture"), to which Indenture and all further instruments supplemental
thereto reference is hereby made for a description of the properties mortgaged
and pledged, the nature and extent of the security, the rights of the holders
of said Bonds and the coupons appurtenant to coupon Bonds and of the Trustee
and of the Company in respect of such security, and the terms and conditions
upon which said Bonds are and are to be issued and secured.


         To the extent permitted by the Indenture and as provided therein, with
the consent of the Company and upon the written consent or affirmative vote of
at least sixty-six and two-thirds per cent, in principal amount of the Bonds
then outstanding and entitled to consent, and of not less than sixty-six and
two-thirds per cent, in principal amount of the Bonds then outstanding and
entitled to consent of each series affected thereby in case one or more but
less than all of the series of Bonds issued under the Indenture are so
affected, the rights and obligations of the Company and of the holders of Bonds
and coupons appurtenant to coupon Bonds, and the terms and provisions of the
Indenture and of any instrument supplemental thereto may be modified from time
to time, provided that no such modification or alteration shall be made which
would postpone the date fixed herein or in the coupons or in the Indenture for
the payment of the principal of, or any installment of interest on, the Bonds,
or reduce the principal of, or the rate of interest payable on, the Bonds, or
reduce the percentage of the principal amount of Bonds the consent of which is
required for the authorization of any such modification or alteration, or which
would modify, without the written consent of the Trustee, the rights, duties or
immunities of the Trustee.

        As provided in the Indenture, said Bonds are issuable in series which
may vary as in the indenture provided or permitted. This Bond is one of a
series of Bonds entitled "First Mortgage Bonds, Series X, 8.375%" due October
1, 1998.
<PAGE>
 
                                       11



        The Bonds of this Series are subject to redemption, in whole or in
part, at the option of the Company on (but not before) October 1, 1994 or at
any time thereafter, upon at least thirty (30) days' and not more than fifty
(50) days' notice mailed to all registered owners of Bonds of this Series to be
redeemed, at their addresses as the same shall appear on the Bond register of
the Company, all subject to the conditions and as more fully provided in the
Indenture, upon payment of the applicable percentage of the principal amount
thereof set forth below:

<TABLE>
<CAPTION>

       If Redeemed During 12
       Month Period Beginning
             October 1           Percentage
       ----------------------    ----------
       <S>                       <C>
                1994              102.284
                1995              101.523
                1996              100.761
                1997              100.000

</TABLE>

together, in each case, with accrued interest to the redemption date.

        The Bonds of Series X are subject to redemption, pursuant to the terms
of the mandatory sinking fund provided in the Indenture, on October 1, 1990 and
on each October 1 thereafter, at 100% of the principal amount thereof plus
accrued interest to the redemption date in the following respective principal
amounts:

<TABLE> 
<CAPTION>
 
                    October 1 of       Principal       October 1 of   Principal
                      the Year           Amount          the Year       Amount
                    ------------       ---------       ------------   ---------
                    <S>                  <C>               <C>        <C>
                        1990             $665,000           1994       $665,000
                        1991              665,000           1995        665,000
                        1992              665,000           1996        665,000
                        1993              665,000           1997        665,000
 
</TABLE>

        The Bonds of this Series X are further subject to redemption in whole
or in part by application of monies deposited with the Trustee in certain cases
for the release of properties from the lien of the Indenture, all subject to the
conditions and as more fully set forth in the Indenture, at any time upon
notice at 100% of the principal amount thereof, together, in each case, with
accrued interest to the redemption date.

        The particular Bonds of Series X to be redeemed from time to time shall
be selected by the Trustee in the amount of $1,000 or a multiple thereof and
<PAGE>
 
                                       12

as nearly as practicable pro rata according to the respective principal
amounts of such Bonds.

        If this Bond or any portion thereof ($1,000 or a multiple) shall be
duly called for redemption as provided in the Indenture, this Bond or such
portion thereof shall (unless the Company shall default in the payment of the
redemption price) cease to bear interest from and after the date fixed for
redemption.

        Upon any partial redemption of this Bond, this Bond may, at the option
of the registered holder hereof, be either (a) surrendered to the Trustee in
exchange for one or more new Bonds of this Series for the principal amount of
the unredeemed portion of this Bond or (b) submitted to the Trustee for
notation hereon by the Trustee of the payment of the portion of the principal
hereof so called for redemption.

        If an Event of Default, as defined in the Indenture, shall occur, the
principal of this Bond may become or be declared due and payable, in the manner
and with the effect provided in the Indenture.

        This Bond is transferable by the registered owner hereof in person or
by attorney authorized in writing, at said principal corporate trust office of
the Trustee, upon surrender for cancellation of this Bond and on payment of
charges, and upon any such transfer a new Bond or Bonds, of the same series,
for the same aggregate principal amount, will be issued to the transferee in
exchange herefor.

        First Mortgage Bonds, Series X, 8.375%, due October 1, 1998, are
issuable as fully registered Bonds without coupons of the denominations of
$1,000 and any multiple of $1,000 which may be executed by the Company and
delivered to the Trustee for authentication and delivery. Bonds of said series
upon surrender thereof to the Trustee at this principal corporate trust office
in the Borough of Manhattan, The City of New York, are exchangeable for other
Bonds of the same series in such authorized denomination or denominations in
the same aggregate principal amount, as may be requested by the holders
surrendering the same.

        The Company and the Trustee may deem and treat the person in whose name
this Bond is registered as the absolute owner hereof, for the purpose of
receiving payment of or on account of the principal hereof and interest due
<PAGE>
 
                                       13


hereon, and neither the Company nor the Trustee shall be affected by any notice
to the contrary.

        No recourse shall be had for the payment of the principal of or the
interest on this Bond, or for any claim based hereon or otherwise in respect
hereof or of the Indenture or of any indenture supplemental thereto, against
any incorporator, stockholder, director or officer, as such, past, present or
future, of the Company or of any predecessor or successor corporation, either
directly or through the Company or any predecessor or successor corporation,
whether by virtue of any constitution, statute or rule of law, or by the
enforcement of any assessment or penalty or by any legal or equitable
proceeding or otherwise howsoever; all such liability being, by the acceptance
hereof and as a part of the consideration for the issuance hereof, expressly
waived and released by every holder hereof, as more fully provided in the
Indenture; provided, however, that nothing herein or in the Indenture contained
shall be taken to prevent recourse to and the enforcement of the liability, if
any, of any shareholder or any stockholder or subscriber to capital stock upon
or in respect of shares of capital stock not fully paid up.


        This Bond shall not be valid or become obligatory for any purpose until
the certificate of authentication hereon shall have been signed by the Trustee,
or its successor as Trustee, under the Indenture.
<PAGE>
 
                                       14

        IN WITNESS WHEREOF, the Company has caused this Bond to be signed in
its name by its President or one of its Vice-Presidents, and its corporate seal
to be impressed or imprinted hereon and attested by its Secretary or one of its
Assistant Secretaries.

 Dated,

                                                     BLACK HILLS CORPORATION,

                                                     By ----------------------
                                                             President
ATTEST:

 
- -----------------------
     Secretary

               (FORM OF TRUSTEE'S CERTIFICATE OF AUTHENTICATION)


        This is one of the Bonds, of the series designated therein, described
in the within mentioned Indenture.

                                                  MANUFACTURERS HANOVER
                                                   TRUST COMPANY, As Trustee,


                                                  By ------------------------
                                                        Authorized Officer
<PAGE>
 
                                       15



        SECTION 1.03. The Bonds of Series X shall be redeemable (except as
otherwise provided in the last paragraph of this Section 1.03 or in Section
2.02 of this Supplemental Indenture) at the option of the Company, at any time
and from time to time, in whole or in part, in the manner and upon the notice
provided in Article Ten of the Indenture, at the redemption prices, and subject
to the conditions, set forth in the form of Bond of Series X provided in
Section 1.02 of this Supplemental Indenture, together, in each case, with
accrued interest to the redemption date.

        Any monies applied to the redemption of Bonds of Series X pursuant to
the provisions of Section 8.08(b) of the Indenture shall be so applied at a
redemption price equal to 100% of the principal amount thereof, together, in
each case, with accrued interest to the redemption date.

        SECTION 1.04. Notwithstanding the provisions of Section 10.03 of the
Indenture, in case of the redemption at any time of less than all the
outstanding Bonds of Series X, the particular Bonds or parts thereof to be
redeemed shall be selected by the Trustee from the outstanding Bonds of Series
X not previously called for redemption as nearly as practicable pro rata
according to the respective principal amounts of such Bonds, provided that the
portions of the principal of Bonds of Series X at any time so selected for
redemption in part shall be equal to $1,000 or a multiple thereof.

                                 ARTICLE TWO
                      SINKING FUND FOR BONDS OF SERIES X

        SECTION 2.01. As a sinking fund for the retirement of Bonds of Series
X, so long as any of the Bonds of said series shall be outstanding, the Company
will deposit with the Trustee on September 30, 1990 and annually thereafter on
each September 30 to and including September 30, 1997 (each such date being
herein sometimes referred to as a "Series X Sinking Fund Payment Date"), cash
in an amount sufficient for the redemption of $665,000 aggregate principal
amount of Bonds of Series X on the next ensuing October 1 at a redemption price
of 100% of the principal amount thereof, together, in each case, with accrued
interest to the redemption date, and thereupon the Trustee shall apply such
cash to the redemption of said aggregate principal amount of the Bonds of said
series on said next ensuing October 1. Any redemption of less than all of the
Bonds of Series X shall
<PAGE>
 
                                       16


not relieve the Company of its obligation to redeem Bonds of Series X in
accordance with the requirements of this Section 2.01.

        SECTION 2.02. Whenever the Trustee shall be required to redeem Bonds
pursuant to the provisions of Section 2.01 of this Supplemental Indenture, the
Trustee shall, on or before the 45th day prior to the Series X Sinking Fund
Payment Date, proceed to select for redemption, from the Bonds of said series,
in the manner provided in Section 1.04 of this Supplemental Indenture, the
aggregate principal amount of Bonds of said series required by the provisions
of said Section 2.01 to be redeemed by application of the cash to be paid to
the Trustee on said Series X Sinking Fund Payment Date, and for and on behalf
of the Company and in the name of the Company, the Trustee shall give notice,
as required by the provisions of Article Ten of the Indenture, of the
redemption for the Series X Sinking Fund of the Bonds so selected. Subject to
the provisions of this Article, the redemption of such Bonds shall be effected
in the manner and upon the terms provided in Section 10.03 of the Indenture at
the sinking fund redemption price of 100% of the principal amount thereof,
together, in each case, with accrued interest to the redemption date.


                                ARTICLE THREE
                           MISCELLANEOUS PROVISIONS

        SECTION 3.01. The Company covenants that so long as any bonds of Series
X shall remain outstanding it will comply with the covenants contained in
Sections 9.06, 9.15, 9.16, 9.20 and 9.22 of the Indenture.

        SECTION 3.02. Prior to the authentication and delivery of the Bonds of
Series X, the Company shall redeem the $5,790,000 of Series P Bonds. The
aggregate principal amount of Bonds which, immediately after the authentication
and delivery of the Bonds of Series X to be issued under this Supplemental
Indenture, will be outstanding under the provisions of, and secured by, the
Indenture, as amended by this Supplemental Indenture, will be $31,182,000,
consisting of the Bonds of Series H through Series O and Series R through
Series W hereinbefore set forth in the second recital of this Supplemental
Indenture and $6,000,000 aggregate principal amount of Bonds of Series X hereby
created.
<PAGE>
 
                                       17

        SECTION 3.03. The Company, by the execution hereof, acknowledges that a
true copy of this Supplemental Indenture has been delivered to and received by
it.

        SECTION 3.04. Except as amended by this Supplemental Indenture, all the
provisions, terms and conditions of the Indenture shall continue in full force
and effect.

        SECTION 3.05. This Supplemental Indenture may be executed in several
counterparts, all or any of which may be treated for all purposes as one
original and shall constitute and be one and the same instrument.
<PAGE>
 
                                       18




        IN WITNESS WHEREOF, BLACK HILLS CORPORATION, party hereto of the first
part, has caused this Supplemental Indenture to be executed on its behalf by
its Chairman of the Board or its President or one of its Vice Presidents and
its corporate seal to be hereto affixed and to be attested by its Secretary or
an Assistant Secretary, and MANUFACTURERS HANOVER TRUST COMPANY, party hereto
of the second part, in evidence of its acceptance of the trust hereby created,
has caused this Supplemental Indenture to be executed on its behalf by one of
its Vice Presidents or Assistant Vice Presidents and its corporate seal to be
hereto affixed and to be attested by its Secretary or an Assistant Secretary,
all as of the day and year first above written.

                                                  BLACK HILLS CORPORATION,
[Corporate Seal]
                                                  By   GEORGE T. LOCKE
                                                     ------------------------
                                                Senior Vice President-Finance
ATTEST:

        DAVID E. MORRILL
- -------------------------------
Assistant Secretary.

Signed, sealed and delivered by
 BLACK HILLS CORPORATION in the
 presence of:

   BONITA A. EDWARDS
- -------------------------------

     JODI HANSON
- -------------------------------
                                                MANUFACTURERS HANOVER
                                                 TRUST COMPANY,
 [Corporate Seal]
                                                By    E. F. COCKINGS
                                                  ----------------------
                                                Assistant Vice President
ATTEST:

    P. FERRERI
- -------------------------------
  Assistant Secretary.

Signed, sealed and delivered by
 MANUFACTURERS HANOVER TRUST COMPANY in the
 presence of:

     T. C. KNIGHT
- -------------------------------

    J. T. BALDUCCI
- -------------------------------
<PAGE>
 
                                       19


STATE OF SOUTH DAKOTA
COUNTY OF PENNINGTON          SS:


        On this 17th day of September, 1986 before me, Connie J. Dulany, the
undersigned officer, personally appeared George T. Locke, to me personally
known, who acknowledged himself to be, and being by me duly sworn, did say that
he is Senior Vice President--Finance of BLACK HILLS CORPORATION, a corporation,
and that the seal affixed to the foregoing instrument is the corporate seal of
said corporation and that said instrument was executed by, and signed in the
name of, the corporation, by him, as such Senior Vice President--Finance and
sealed in behalf of the corporation by authority of its Board of Directors for
the purposes therein contained, and the said George T. Locke acknowledged the
same as the free act and deed of said corporation.

       IN WITNESS WHEREOF, I hereunto set my hand and official seal.

[Notarial Seal]
                                                     CONNIE J. DULANY
                                           -----------------------------------
                                                       Notary Public
                                           My commission expires July 13, 1994
<PAGE>
 
                                       20

STATE OF NEW YORK
COUNTY OF NEW YORK    SS:

        On this 18th day of September, 1986 before me, Kathy A. Murphy, the
undersigned officer, personally appeared E. F. Cockings, to me personally
known, who acknowledged himself to be, and being by me duly sworn, did say that
he is Assistant Vice President of MANUFACTURERS HANOVER TRUST COMPANY, a
corporation, and that the seal affixed to the foregoing instrument is the
corporate seal of said corporation and that said instrument was executed by,
and signed in the name of, the corporation, by him, as such Vice President, and
sealed in behalf of the corporation by authority of its Board of Directors for
the purposes therein contained, and the said E. F. Cockings acknowledged the
same as the free act and deed of said corporation.

        IN WITNESS WHEREOF, I hereunto set my hand and official seal.

[Notarial Seal]
                                                  KATHY A. MURPHY
                                       ------------------------------------
                                                   Notary Public
                                                  Kathy A. Murphy
                                         Notary Public, State of New York
                                                  No. 60-482 5140
                                          Qualified in Westchester County
                                       Certificate filed in New York County
                                         Commission expires July 31, 1988.
<PAGE>
 
                                       21

        Counterparts of the foregoing Twenty Third Supplemental Indenture were
recorded or filed as follows:
South Dakota Secretary of State:
  Counterpart No. 1 recorded 9/22/86 at 4:00 P.M. in Volume 28, Page 8
  Counterpart No. 2 filed 9/22/86 at 10:25 A.M., Document No. 346852
Nebraska Secretary of State:
  Counterpart No. 3 filed 9/22/86 at 3:08 P.M., Document No. 355082
Montana Secretary of State:
  Counterpart No. 4 filed 9/22/86 at 10:39 A.M., Filing No. 236638
Wyoming Secretary of State:
  Counterpart No. 5 filed 9/22/86 at 9:00 A.M., File No. 14831
Dawes County, Nebraska:
  Counterpart No. 6 recorded 9/19/86 at 2:00 P.M. in Book 102,
  Page 1-20 Docket No. 22
  Counterpart No. 7 filed 9/19/86 at 2:00 P.M., File No. 1002
Scotts Bluff County, Nebraska:
  Counterpart No. 8 recorded 9/19/86 at 3:45 P.M. in General Index for
  Mortgages Book 262, Page 556, Document No. 3950
  Counterpart No. 9 recorded 9/19/86 at 3:45 P.M. in Book 1 of Financing
  Statements, Page 984
Sioux County, Nebraska:
  Counterpart No. 10 recorded 9/19/86 at 1:40 P.M. in Book 45 of Mortgages,
  Page 285-305
  Counterpart No. 11 filed 9/19/86 at 1:45 P.M., Filing No. 6062
Carter County, Montana:
  Counterpart No. 12 recorded 9/19/86 at 2:30 P.M. in Book 28 of
  Mortgages on Pages 1-21, Docket No. 159556
  Counterpart No. 13 filed 9/19/86 at 2:30 P.M., File No. 159557 and 4524
Powder River County, Montana:
  Counterpart No. 14 recorded 9/19/86 at 2:20 P.M., Book 36MF of Mortgages,
  Page 647, Docket No. 113122
  Counterpart No. 15 filed 9/19/86 at 2:20 P.M., File No. 113121
Campbell County, Wyoming:
  Counterpart No. 16 recorded 9/19/86 at 3:50 P.M. in Book 906 of Photos,
  Page 229-249, Docket No. 588104
  Counterpart No. 17 filed 9/19/86 at 1:22 P.M., File No. 4128961
Crook County, Wyoming:
  Counterpart No. 18 recorded 9/19/86 at 1:10 P.M. in Book 252 of Photos,
  Page 777, Docket No. 454725
  Counterpart No. 19 filed 9/19/86 at 1:10 P.M., File No. 21918
Weston County, Wyoming:
  Counterpart No. 20 recorded 9/19/86 at 2:30 P.M., Book 153 of Photos,
  Page 654-674, Docket No. 526160
  Counterpart No. 21 filed 9/19/86 at 2:30 P.M., File No. 038336 and 526161


<PAGE>
 
                                                                 Conformed Copy
    
- -------------------------------------------------------------------------------

                           BLACK HILLS CORPORATION

                                      TO

                     MANUFACTURERS HANOVER TRUST COMPANY,

  
                                                         As Trustee
                               

                            ---------------------
                               
                                TWENTY FOURTH


                            SUPPLEMENTAL INDENTURE


                          Dated as of April 13, 1987

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


                  Supplemental to Indenture of Mortgage and


                 Deed of Trust Dated as of September 1, 1941


- -------------------------------------------------------------------------------
<PAGE>
 
    TWENTY FOURTH SUPPLEMENTAL INDENTURE, dated as of the 13th day of 
April, 1987, between Black Hills Corporation (formerly named and now 
operating its electric utility division as Black Hills Power and Light 
Company), a corporation organized and existing under the laws of the State 
of South Dakota (hereinafter called the "Company"), party of the first 
part, and Manufacturers Hanover Trust Company, a corporation organized and 
existing under the laws of the State of New York, as Trustee under the 
Indenture hereinafter mentioned (hereinafter called the "Trustee"), party 
of the second part.

    WHEREAS, in order to secure an authorized issue of First Mortgage 
Bonds of the Company, the Company has executed and delivered an Indenture 
of Mortgage and Deed of Trust to Central Hanover Bank and Trust Company 
(subsequently known as The Hanover Bank) as Trustee, dated September 1, 
1941, hereinafter referred to as the "Original Indenture", and has also 
executed and delivered to said Trustee and to Manufacturers Hanover Trust 
Company (which on September 8, 1961 became the Trustee under the Original 
Indenture, as theretofore supplemented and amended, by virtue of the merger 
of said The Hanover Bank into Manufacturers Trust Company, under said name 
Manufacturers Hanover Trust Company), as Trustee, various Supplemental 
Indentures supplementing and/or modifying the Original Indenture, 
respectively dated as follows:


<TABLE>

<S>                                         <C>
    First Supplemental Indenture               July 15, 1945
    Second Supplemental Indenture              January 15, 1948
    Third Supplemental Indenture               January 15, 1949
    Fourth Supplemental Indenture              March 1, 1950
    Fifth Supplemental Indenture               March 1, 1952
    Sixth Supplemental Indenture               July 1, 1956
    Seventh Supplemental Indenture             May 1, 1957
    Eighth Supplemental Indenture              May 1, 1959
    Ninth Supplemental Indenture               April 1, 1960
    Tenth Supplemental Indenture               August 1, 1960
    Eleventh Supplemental Indenture            June 1, 1961
    Twelfth Supplemental Indenture             October 1, 1962
    Thirteenth Supplemental Indenture          May 1, 1963
    Fourteenth Supplemental Indenture          June 1, 1969
    Fifteenth Supplemental Indenture           June 15, 1974
    Sixteenth Supplemental Indenture           August 1, 1974
    Seventeenth Supplemental Indenture         July 15, 1975
    Eighteenth Supplemental Indenture          May 1, 1976
    Nineteenth Supplemental Indenture          February 15, 1977  
    Twentieth Supplemental Indenture           April 1, 1977
    Twenty First Supplemental Indenture        June 1, 1977
    Twenty Second Supplemental Indenture       July 14, 1982
    Twenty Third Supplemental Indenture        September 1, 1986

</TABLE>
    
and the Company also executed and delivered to said The Hanover Bank, as 
Trustee, Subordination Agreements dated December 29, 1950 and September 19,
<PAGE>
 
                                       2



1955 also supplementing the Original Indenture, which, as supplemented and 
amended by said twenty-three Supplemental Indentures and said Subordination 
Agreements, is hereinafter referred to as the "Indenture"; and

    WHEREAS, pursuant to the provisions of the Indenture, First Mortgage 
Bonds have been duly issued and are presently outstanding and secured by
the Indenture as follows:

<TABLE>  
<CAPTION>
                                                 Principal Amount
                    Series                          Outstanding
                    ------                       ----------------
<S>                                             <C>
    Series H, 5-1/4%, due May 15, 1987               $ 634,000
    Series I, 5-1/8%, due May 1, 1989                  865,000
    Series J, 5-3/4%, due April 1, 1990                870,000
    Series K, 5-5/8%, due August 1, 1990               870,000
    Series L, 5-3/8%, due June 1, 1991                 875,000
    Series M, 5%, due October 1, 1992                  875,000
    Series N, 4-3/4%, due May 1, 1993                  884,000
    Series O, 8.05%, due June 1, 1999                4,975,000
    Series R, 6-5/8%, due April 1, 2007              5,850,000
    Series S, 6-5/8%, due April 1, 2007              2,050,000
    Series T, 6-5/8%, due April 1, 2007              1,000,000
    Series U, 6-5/8%, due April 1, 2007              1,000,000
    Series V, 6.85%, due June 1, 2007                1,550,000
    Series W, 6.85%, due June 1, 2007                2,850,000
    Series X, 8.375%, due October 1, 1998            6,000,000
                                                   -----------
                                                   $31,148,000
                                                   -----------
</TABLE>  

; and

    WHEREAS, Section 9.21 of the Original Indenture provides a covenant 
requiring the Company upon the acquisition of any shares of stock of any 
Subsidiary to deposit and pledge the stock of such Subsidiary to the 
Trustee subject to the direct lien of the Indenture and further prohibits 
the Company from sanctioning or permitting such Subsidiary from acquiring 
any indebtedness in excess of $15,000 except indebtedness issued to the 
Company; and

    WHEREAS, Section 9.21 of the Original Indenture has been amended to 
specifically except Wyodak Resources Development Corp. (defined herein as 
"Wyodak Resources"), the Company's wholly owned Subsidiary, from the 
requirements and restrictions of Section 9.21 of the Indenture thereby 
allowing Wyodak Resources to acquire or create its Subsidiaries free from 
the requirements and restrictions of Section 9.21; and

    WHEREAS, the Company believes that in properly conducting its business 
affairs that it should be allowed to acquire or create Subsidiaries, other 
than Subsidiaries of Wyodak Resources, to hold property and to engage in 
<PAGE>
 
                                       3

businesses unrelated to the electric utility business affairs of the 
Company and free of any restrictions of the Indenture and to be able to 
incur indebtedness other than indebtedness issued to the Company and to 
encumber such Subsidiaries' stock without any pledge to the lien of the 
Indenture; and

    WHEREAS, since the Indenture as supplemented now places no 
restrictions on Wyodak Resources' Subsidiaries incurring indebtedness and 
keeping the stock of such Subsidiaries free of the lien of the Indenture, 
to allow through this Supplemental Indenture for the Company to acquire or 
create Subsidiaries other than Subsidiaries of Wyodak Resources free of the 
same restrictions would not have any effect on the security of the 
Indenture, and as long as the Company is limited from making investments or 
advances in or to Subsidiaries from only those funds from which the Company 
may pay common stock dividends or any other distribution in respect 
thereto, that an amendment of the Indenture which removes Subsidiaries from 
the requirements and restrictions of Section 9.21 of the Indenture and 
includes Subsidiaries in the dividend restriction provisions of Section 
9.16 of the Indenture is consistent with the intent of the Bondholders in 
approving the above referred to amendments to Sections 9.16 and 9.21 of the 
Indenture and does not impair the security of the Indenture; and

    WHEREAS, the Company deems it advisable that the Indenture be amended 
as herein provided and the holder of the requisite amount of the 
outstanding bonds issued under the Indenture have duly consented, pursuant 
to the provisions of Section 18.11 of the Indenture, to the execution of 
this Twenty Fourth Supplemental Indenture and have lodged with the Trustee 
an instrument duly setting forth such consent; and

    WHEREAS, the Company, in the exercise of the powers and authority 
conferred upon and reserved to it under and by virtue of the provisions of 
the Original Indenture, and particularly the provisions contained in 
Articles Seventeen and Eighteen thereof, and pursuant to appropriate 
resolutions of its Board of Directors has duly resolved and determined to 
make, execute and deliver to the Trustee the Twenty Fourth Supplemental 
Indenture in the form hereof for the purposes therein provided; and

    WHEREAS, all conditions and requirements necessary to make this 
Twenty Fourth Supplemental Indenture a valid, binding and legal instrument 
in accordance with its terms have been done, performed and fulfilled, and 
the execution and delivery hereof have been in all respects duly 
authorized.
    
    NOW, THEREFORE, THIS INDENTURE WITNESSETH:
     
    That the Company, in consideration of the premises and one dollar to 
it duly paid by the Trustee at or before the ensealing and delivery of 
these presents, the receipt whereof is hereby acknowledged, and of other 
good and valuable consideration, in order to amend certain provisions of 
the Indenture, hereby further covenants and agrees to and with the Trustee 
and its successors in the trust under the Indenture for the benefit of all 
those who shall from time to time hold the Bonds as follows:
                                                                  
<PAGE>
 
                                       4


                                 ARTICLE ONE

                         Amendments to the Indenture

                  
    SECTION 1.01 Subparagraph (6) of Section 9.16 of Article Nine as set 
forth in Section 1.06 of the Fifteenth Supplemental Indenture dated June 
15, 1974 is hereby amended as follows:

    "(6) The aggregate of all investments or advances by the Company in 
         or to any Subsidiary."
        
    SECTION 1.02. Section 9.21 of Article Nine as previously amended by 
Section 1.07 of the Fifteenth Supplemental Indenture dated June 15, 1974 
and by Section 1.06 of the Eighteenth Supplemental Indenture dated May 1, 
1976 and by Section 1.03 of the Twenty Second Supplemental Indenture dated 
July 14, 1982, is further amended by deleting the first two paragraphs 
thereof and amending the third paragraph so that the entire Section 9.21 is 
amended to read as follows:

    "The Company shall make no investment in any Subsidiary 
    unless as of the date of the making of an investment therein 
    the amount of such investment when added to the aggregate of 
    all investments made prior thereto by the Company in 
    Subsidiaries shall not be in excess as of such date of 
    twenty per cent of the total of (1) the capital stock 
    liability of the Company plus (or minus in the case of a 
    deficit) the capital surplus and earned surplus of the 
    Company and (2) any premium on the capital stock of the 
    Company not included in surplus."
   

                                 ARTICLE TWO
 
                           Miscellaneous Provisions
                   
    SECTION 2.01. The Company, by the execution hereof, acknowledges that 
a true copy of this Supplemental Indenture has been delivered to and 
received by it.

    SECTION 2.02. Except as heretofore amended and as amended by this 
Supplemental Indenture, all the provisions, terms and conditions or the 
Original Indenture shall continue in full force and effect.

    SECTION 2.03. The recitals contained in this Supplemental Indenture 
shall be taken as the statements of the Company, and the Trustee assumes 
no responsibility for their correctness.

    SECTION 2.04. This Supplemental Indenture may be executed in several 
counterparts, all or any of which may be treated for all purposes as one 
original and shall constitute and be one and the same instrument.
                                                                 
<PAGE>
 
                                       5

    IN WITNESS WHEREOF, BLACK HILLS CORPORATION, party hereto of the 
first part, has caused this Supplemental Indenture to be executed on its 
behalf by its President or one of its Vice Presidents and its corporate 
seal to be hereto affixed and to be attested by its Secretary or an 
Assistant Secretary, and MANUFACTURERS HANOVER TRUST COMPANY, party hereto 
of the second part, in evidence of its acceptance of the trust hereby 
created, has caused this Supplemental Indenture to be executed on its 
behalf by one of its Vice Presidents or Assistant Vice Presidents and its 
corporate seal to be hereto affixed and to be attested by its Secretary or 
an Assistant Secretary, all as of the day and year first above written.

                                       BLACK HILLS CORPORATION
(CORPORATE SEAL)


                                       By GEORGE T. LOCKE
                                          -------------------------------------
                                       Its Senior Vice President - Finance
                                           -------------------------------------

ATTEST:

          LOUISE S. KELLEY
- --------------------------------------
              Secretary
   
Signed, sealed and delivered 
by BLACK HILLS CORPORATION in 
the presence of

          ROXANN R. BASHAM
- --------------------------------------

           LINDA L. MEYER
- --------------------------------------

                                                                
<PAGE>
 
                                       6

                                       MANUFACTURERS HANOVER TRUST COMPANY

                                       By       ERNIE F. COCKINGS
                                          -------------------------------------
                                       Its Assistant Vice President
                                           -------------------------------------

ATTEST:


           MARK G. WALSH
- --------------------------------------
Its Trust Officer
    ----------------------------------

Signed, sealed and delivered by
MANUFACTURERS HANOVER TRUST 
COMPANY in the presence of:

          ELEANOR T. GREEN
- --------------------------------------

          MICHAEL F. KIELY
- --------------------------------------

   
STATE OF SOUTH DAKOTA  )
                       ) SS.
COUNTY OF PENNINGTON   )

    On this 13th day of April, 1987, before me, Jodi Hanson, the undersigned
            ----        -----                   -----------
officer, personally appeared George T. Locke, to me personally known, who
                             ---------------
acknowledged himself to be, and being by me duly sworn, did say that he is
Senior Vice President - Finance of BLACK HILLS CORPORATION, a corporation, and
- -------------------------------
that the seal affixed to the foregoing instrument is the corporate seal of said
corporation and that said instrument was executed by, and signed in the name of,
the corporation, by him, as such Senior Vice President - Finance and sealed in
                                 -------------------------------
behalf of the corporation by authority of its Board of Directors for the
purposes therein contained, and the said Senior Vice President - Finance
                                         -------------------------------
acknowledged the same as the free act and deed of said corporation.

    IN WITNESS WHEREOF, I hereunto set my hand and official seal.
    


                                                     JODI HANSON
                                       ----------------------------------------
                                       NOTARY PUBLIC
 (SEAL)                                My Commission expires June 24, 1993
<PAGE>
 
                                       7


STATE OF NEW YORK     )
                      ) SS.
COUNT OF NEW YORK     )

    On this 13th day of May, 1987, before me, Kathy A. Murphy, the undersigned
            ----        ---                   ---------------
officer, personally appeared E. F. Cockings, to me personally known, who
                             --------------
acknowledged himself to be, and being by me duly sworn, did say that he is
Assistant Vice President of MANUFACTURERS HANOVER TRUST COMPANY, a corporation,
- ------------------------
and that the seal affixed to the foregoing instrument is the corporate seal of
said corporation and that said instrument was executed by, and signed in the
name of, the corporation, by him, as such Assistant Vice President and sealed
                                          ------------------------
in behalf of the corporation by authority of its Board of Directors for the
purposes therein contained, and the said E. F. Cockings acknowledged the same as
                                         --------------
the free act and deed of said corporation.

    IN WITNESS WHEREOF, I hereunto set my hand and official seal.
    
                                                    KATHY A. MURPHY
                                       -----------------------------------------
                                       NOTARY PUBLIC
(SEAL)
                                       Kathy A. Murphy
                                       Notary Public, State of New York
                                       No. 60-4825140
                                       Qualified in Westchester County
                                       Certificate filed in New York County
                                       Commission Expires July 31, 1988
<PAGE>
 
                                       8

    Counterparts of the foregoing Twenty Fourth Supplemental 
Indenture were recorded or filed as follows:

South Dakota Secretary of State:
    Counterpart No. 3 recorded 6/15/87 at 4:00 P.M. in Volume 28, Page 13 of
     Railway Deeds, Mortgages and Leases Record Book

Dawes County, Nebraska:
    Counterpart No. 4 recorded 6/15/87 at 11:35 A.M. in Book 102, Page 1-8
     Docket No. 23
     
Scotts Bluff County, Nebraska:
    Counterpart No. 5 recorded 6/15/87 at 1:35 P.M. in General Index for
     Mortgages Book 119, Pages 216-223, Document No. 2961
     
Sioux County, Nebraska:
    Counterpart No. 6 recorded 6/15/87 at 8:10 A.M. in Book 45 of Mortgages,
     Pages 718-725
     
Carter County, Montana:
    Counterpart No. 7 recorded 6/16/87 at 8:00 A.M. in Book 28 of Mortgages, on
     Pages 209-216, Docket No. 160224
     
Powder River County, Montana:
    Counterpart No. 8 recorded 6/15/87 at 10:02 A.M., Book 39MF of Mortgages,
     Pages 14-21, Docket No. 114168
    
Campbell County, Wyoming:
    Counterpart No. 9 recorded 6/15/87 at 9:22 A.M. in Book 954 of Photos, Pages
     459-466, Docket No. 599912
    
Crook County, Wyoming:
    Counterpart No. 10 recorded 6/15/87 at 9:00 A.M. in Book 262 of Photos,
     Pages 344-351, Docket No. 460555
    
Weston County, Wyoming:
    Counterpart No. 11 recorded 6/15/87 at 4:15 P.M., Book 161 of Photos, Pages
     24-31, Docket No. 532965    

<PAGE>
 
                                                                 Conformed Copy
                                        

================================================================================
 
                                        
                           BLACK HILLS CORPORATION

                                      TO

                     MANUFACTURERS HANOVER TRUST COMPANY,

                   

                                                           As Trustee
    
                            ----------------------
                                           
                                           
                                 TWENTY FIFTH


                  
                            SUPPLEMENTAL INDENTURE
 

                          Dated as of June 15, 1988


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

                  Supplemental to Indenture of Mortgage and


                 Deed of Trust Dated as of September 1, 1941


                       First Mortgage Bonds, Series Y,
                          9.49%, due June 15, 2018


================================================================================
 
<PAGE>
 
             TWENTY FIFTH SUPPLEMENTAL INDENTURE, dated as of the 15th day of
June, 1988, between Black Hills Corporation (which was formerly named Black
Hills Power and Light Company and which now operates its electric utility
operations under the assumed name of Black Hills Power and Light Company), a
corporation organized and existing under the laws of the State of South Dakota
(hereinafter called the "Company"), party of the first part, and Manufacturers
Hanover Trust Company, a corporation organized and existing under the laws of
the State of New York, as Trustee under the Indenture hereinafter mentioned
(hereinafter called the "Trustee"), party of the second part.

             WHEREAS, in order to secure an authorized issue of First Mortgage
Bonds of the Company, the Company has executed and delivered an Indenture of
Mortgage and Deed of Trust to Central Hanover Bank and Trust Company
(subsequently known as The Hanover Bank), as Trustee, dated September 1, 1941,
hereinafter referred to as the "Original Indenture", and has also executed and
delivered to said Trustee and to Manufacturers Hanover Trust Company (which on
September 8, 1961 became the Trustee under the Original Indenture, as 
theretofore supplemented and amended, by virtue of the merger of said The
Hanover Bank into Manufacturers Trust Company, under said name Manufacturers
Hanover Trust Company), as Trustee, various Supplemental Indentures 
supplementing and/or modifying the Original Indenture, respectively dated as
follows:

          First Supplemental Indenture              July 15, 1945
          Second Supplemental Indenture             January 15, 1948
          Third Supplemental Indenture              January 15, 1949
          Fourth Supplemental Indenture             March 1, 1950
          Fifth Supplemental Indenture              March 1, 1952
          Sixth Supplemental Indenture              July 1, 1956
          Seventh Supplemental Indenture            May 1, 1957
          Eighth Supplemental Indenture             May 1, 1959
          Ninth Supplemental Indenture              April 1, 1960
          Tenth Supplemental Indenture              August 1, 1960
          Eleventh Supplemental Indenture           June 1, 1961
          Twelfth Supplemental Indenture            October 1, 1962
          Thirteenth Supplemental Indenture         May 1, 1963
          Fourteenth Supplemental Indenture         June 1, 1969
          Fifteenth Supplemental Indenture          June 15, 1974
          Sixteenth Supplemental Indenture          August 1, 1974
          Seventeenth Supplemental Indenture        July 15, 1975
          Eighteenth Supplemental Indenture         May 1, 1976
          Nineteenth Supplemental Indenture         February 15, 1977
          Twentieth Supplemental Indenture          April 1, 1977
          Twenty First Supplemental Indenture       June 1, 1977
          Twenty Second Supplemental Indenture      July 14, 1982
          Twenty Third Supplemental Indenture       September 1, 1986
          Twenty Fourth Supplemental Indenture      April 13, 1987
     
and the Company also executed and delivered to said The Hanover Bank, as
Trustee, Subordination Agreements dated December 29, 1950 and September 19, 1955
also supplementing the Original Indenture, which, as supplemented and amended by
said twenty-four Supplemental Indentures and said Subordination Agreements, is
hereinafter referred to as the "Indenture"; and
<PAGE>
 
            WHEREAS, pursuant to the provisions of the Indenture, First
Mortgage Bonds have been duly issued and are presently outstanding and secured
by the Indenture as follows:
 
<TABLE>
<CAPTION>

                                                                      Principal Amount                                              
      Series                                                             Outstanding                                  
      ------                                                          ----------------
      <S>                                                             <C>
      Series I, 5 1/8%, due May 1, 1989                                   $ 860,000    
      Series J, 5 3/4%, due April 1, 1990                                   865,000    
      Series K, 5 5/8%, due August 1, 1990                                  865,000    
      Series L, 5 3/8%, due June 1, 1991                                    870,000    
      Series M, 5%, due October 1, 1992                                     870,000    
      Series N, 4 3/4%, due May 1, 1993                                     879,000    
      Series O, 8.05%, due June 1, 1999                                   4,975,000    
      Series R, 6 5/8%, due April 1, 2007                                 5,850,000    
      Series S, 6 5/8%, due April 1, 2007                                 2,050,000    
      Series T, 6 5/8%, due April 1, 2007                                 1,000,000    
      Series U, 6 5/8%, due April 1, 2007                                 1,000,000    
      Series V, 6.85%, due June 1, 2007                                   1,550,000    
      Series W, 6.85%, due June 1, 2007                                   2,850,000    
      Series X, 8.375%, due October 1, 1998                               6,000,000
                                                                        -----------    
                                                                        $30,484,000
                                                                        ===========    
</TABLE> 


; and


             WHEREAS, the Company has acquired certain property not heretofore
specifically described under the Indenture and it desires to specifically
describe such property; and

             WHEREAS, as permitted by the Indenture, the Company, by
resolutions of its Board of Directors duly adopted, has determined to create a
new series of bonds to be known as its "First Mortgage Bonds, Series Y, 9.49%,
due June 15, 2018" (herein called "Series Y Bonds"), to be initially
authenticated and delivered in the aggregate principal amount of $6,000,000, in
the form, having the characteristics and being entitled to the benefits as in
this Supplemental Indenture provided; and

             WHEREAS, the Company, in exercise of the powers and authority
conferred upon and reserved to it under and by virtue of the provisions of the
Indenture, and particularly the provisions contained in Articles Two and
Seventeen thereof, and pursuant to appropriate resolutions of its Board of
Directors, has duly resolved and determined to make, execute and deliver to the
Trustee a Twenty Fifth Supplemental Indenture in the form hereof (herein
sometimes referred to as "this Supplemental Indenture") for the purposes herein
provided; and

             WHEREAS, all conditions and requirements necessary to make this
Supplemental Indenture a valid, binding and legal instrument in accordance with
its terms have been done, performed and fulfilled, and the execution and
delivery hereof have been in all respects duly authorized;

                                      -2-


                                                      
<PAGE>
 
           NOW THEREFORE, THIS INDENTURE WITNESSETH: That the Company, in
consideration of the premises and of one dollar to it duly paid by the Trustee
at or before the ensealing and delivery of these presents, the receipt whereof
is hereby acknowledged, and of other good and valuable consideration, in order
to better secure the payment both of the principal of and interest on all Bonds
issued under the Indenture and that may be issued under this or any other
indentures supplemental thereto, according to their tenor and effect, and the
performance by the Company of all the covenants and conditions herein and
therein contained, and in order to establish the terms of the Series Y Bonds,
hereby further covenants and agrees to and with the Trustee and its successors
in the trust under the Indenture for the benefit of all those who shall from
time to time hold the Series Y Bonds as follows:


           The Company does hereby ratify and confirm its Mortgage and Pledge to
the Trustee of all property described in the Indenture and does hereby by these
presents grant, bargain, sell, warrant, alien, remise, release, convey, confirm,
assign, mortgage, pledge and set over unto the Trustee, and to its successors
and assigns forever, the following described property acquired by the Company
and not heretofore specifically described under the Indenture:

                   LANDS IN PENNINGTON COUNTY, SOUTH DAKOTA
           
         Utility Lot 1 of Par Subdivision in the unplatted portion of Northeast
         Quarter Southeast Quarter and Southeast Quarter Southeast Quarter of
         Section 23, Township 1 North, Range 7 East, Black Hills Meridian,
         Containing 0.918 acres, more or less.
         
            TOGETHER with all contracts, agreements, rights and understandings,
whether now owned or hereafter acquired for the purchase or exchange of electric
energy or for the making of connections for exchange of energy or service,
including but not limited to:

         Reserve Capacity Integration Agreement, dated as of May 5, 1987,
         between Pacific Power & Light Company and Black Hills Power and Light 
         Company providing for the integration of the parties reserve capacity
         for a term commencing July 1, 1987 and terminating June 30, 2012.
         
            TOGETHER with, all and singular, the tenements, hereditaments and
appurtenances belonging or in any way appertaining to said property or any part
thereof, with the reversion and reversions, remainder and remainders, tolls,
rents, revenues, issues, earnings, income, products and profits thereof, and all
the estate, right, title and interest and claim whatsoever, at law as well as in
equity, which the Company now has or may hereafter acquire in and to said
property, and every part and parcel thereof;

            TOGETHER with all building, structures and improvements erected or
constructed on any of the above described tracts of land;

            TOGETHER with all property, real, personal and mixed situated upon,
connected with, used in connection with, or in any wise appertaining to any of
the property described in the Indenture or in this Supplemental Indenture,
presently owned by the Company (other than Excepted Property as defined in the
Indenture), and including all rights of way, easements, licenses, permits, and
privileges in connection with or pertaining
 
                                      -3-
                                                           
<PAGE>
 
to all of such property, whether existing by reason of express or implied,
written or oral, evidences of ownership, rights, agreements or understandings;

            TOGETHER with all electric generating plants and electric
transmission and distribution systems now owned by the Company, and any
additions to or extensions of any such electric generating plants and electric
transmission and distribution systems, together with all engines, dynamos,
motors, generators, boilers, turbines, pole lines, poles, wires, cross-arms,
insulators, transformers, meters, buildings, erections, structures, stations,
substations, powerhouses, power producing and power transmitting equipment,
water, water rights, waterwheels, headworks, race-ways, hydraulic works, hydro-
electric plants, cables, conduits, tools, instruments, apparatus, appliances,
machinery, facilities, fixtures and all other property used or provided for use
in the construction, repair, maintenance or operation thereof (other than
Excepted Property as defined in the Indenture), and together also with all the
rights, privileges, franchises, easements, licenses, ordinances, rights of way,
liberties, immunities and permits of the Company howsoever conferred or acquired
with respect to the construction, maintenance, repair or operation of said
electric generating plants and electric transmission or distribution systems,
and each of them, and any additions thereto and extension thereof (EXCEPT, and
not including the Project and Project Site, and all improvements and
appurtenances thereto as defined in and all of which are released from the lien
of the Indenture by the provisions of Article One of the Eighteenth Supplemental
Indenture, dated as of May 1, 1976);

            TO HAVE AND TO HOLD all such properties, real, personal and mixed,
mortgaged, pledged or conveyed by the Company as aforesaid, or intended so to
be, unto the Trustee and its successors and assigns forever;

            IN TRUST, NEVERTHELESS, for the same purposes and upon the same
trusts, terms and conditions, and subject to and with the same provisions and
covenants as are set forth in the Indenture (as amended by this Supplemental
Indenture) with the same effect in all respects as if the property and rights
herein described and herein conveyed to the Trustee had, at the time of the
execution and delivery of the Original Indenture, been owned by the Company and
had been specifically and at length described in and conveyed to the Trustee by
the Original Indenture as a part of the property therein stated to be conveyed.


                                ARTICLE ONE
                           Series Y Bonds, 9.49%
                        
            SECTION 1.01. There is hereby created a series of Bonds, known as
and entitled "First Mortgage Bonds, Series Y, 9.49%, due June 15, 2018", and the
form thereof shall be as provided in this Supplemental Indenture.

            The aggregate principal amount of Series Y Bonds which may be
authenticated and delivered and outstanding under the Indenture and this
Supplemental Indenture shall be limited in aggregate principal amount to
$6,000,000, except as provided under Section 2.12 of the Indenture. The Series Y
Bonds shall bear interest at the rate of 9.49% per annum until the principal
thereof becomes due and payable and thereafter, if default be made in the
payment of such principal, at the rate of 10.49% per annum until the principal
thereof shall be paid. The Series Y Bonds shall mature June 15, 2018.

                                
                                      -4-
                               
                                                      
<PAGE>
 
            The Series Y Bonds shall be registered Bonds without coupons in
denominations of $1,000 and any multiples of $1,000 which may be executed by the
Company and delivered to the Trustee for authentication and delivery. The date
of commencement of the first interest period for the Series Y Bonds shall be the
date of initial authentication and delivery thereof. The Series Y Bonds shall be
dated as provided in Section 2.05 of the Indenture, and shall be numbered from
YRX1 consecutively upwards. All Series Y Bonds shall bear interest from their
respective dates, such interest to be payable semi-annually on the fifteenth day
of June and December in each year. The first interest payment shall be due
December 15, 1988 and shall include interest from the date of initial
authentication and delivery thereof. The principal of, premium, if any, and
interest on the Series Y Bonds shall be payable at the principal corporate trust
office of the Trustee, in the Borough of Manhattan, The City of New York, in any
coin or currency of the United States of America which at the time of payment
shall be legal tender for the payment of public and private debts. The Series Y
Bonds shall be subject to redemption as provided in Sections 1.03 and 1.04 and
Article Two of this Supplemental Indenture.

            The Trustee is hereby appointed Registrar in respect of the Series Y
Bonds, and the principal corporate trust office of the Trustee in the Borough of
Manhattan, The City of New York, is hereby designated as the office or agency of
the Company in said Borough where notices or demands in respect of Series Y
Bonds may be served.

            The definitive Series Y Bonds shall be issued in such printed form
as approved by the Trustee. Subject to the foregoing provisions of this Section
and to the provisions of Section 2.11 of the Indenture, all definitive Series Y
Bonds shall, upon surrender thereof to the Trustee at its principal office, be
exchangeable for other Bonds of the same series, in registered form and in such
authorized denomination or denominations in the same aggregate principal amount,
as may be requested by the holder surrendering the same. The Company will
execute, and the Trustee shall authenticate and deliver, registered Series Y
Bonds without coupons, whenever the same shall be required for any such
exchange.

            SECTION 1.02. The text of the Series Y Bonds, and the certificate of
authentication of the Trustee to be executed thereon, are to be substantially in
the following forms, respectively:

                           (FORM OF SERIES Y BONDS)
                     
            No. YRX __________________                       $ _________________

                            BLACK HILLS CORPORATION
 
            FIRST MORTGAGE BOND, SERIES Y, 9.49% DUE JUNE 15, 2018
        
            BLACK HILLS CORPORATION (hereinafter called the "Company"), a
corporation organized and existing under the laws of the State of South Dakota,
for value received, hereby promises to pay to
or registered assigns, on the 15th day of June, 2018, at the principal corporate
trust office of the Trustee, in the Borough of Manhattan, The City of New York,
_____________________________ Dollars, in any coin or currency
of the United States of America which at the time of payment shall be legal
tender for the payment of public and private debts, and to pay interest thereon
from the date hereof, at the rate of 9.49 per cent, per annum (computed
                                
                                      -5-

                                                       
<PAGE>
 
on the basis of a 360-day year of 12 thirty-day months), payable at said
principal office of the Trustee in like coin or currency semi-annually on June
15 and December 15 in each year until the principal hereof shall have become due
and payable, and thereafter if default be made in the payment of such principal,
at the rate of 10.49 per cent, per annum until the principal hereof shall be
paid. The first interest payment shall be due December 15, 1988 and shall
include interest from the date of initial authentication and delivery thereof.

            This Bond is one of an authorized issue of Bonds of the Company
known as its "First Mortgage Bonds", issued and to be issued in one or more
series under, and all equally and ratably secured (except as any sinking,
amortization, improvement, renewal or other analogous fund, established in
accordance with the provisions of the Indenture hereinafter mentioned, may
afford additional security for the Bonds of any particular series) by an
Indenture of Mortgage and Deed of Trust dated as of September 1, 1941 executed
by the Company to Central Hanover Bank and Trust Company (subsequently known as
The Hanover Bank and herein, with its successor by merger, Manufacturers Hanover
Trust Company, called the "Trustee"), as Trustee, as supplemented and amended by
Supplemental Indentures dated as of July 15, 1945, January 15, 1948, January 15,
1949, March 1, 1950, March 1, 1952, July 1, 1956, May 1, 1957, May 1, 1959, 
April 1, 1960, August 1, 1960, June 1, 1961, October 1, 1962, May 1, 1963, 
June 1, 1969, June 15, 1974, August 1, 1974, July 15, 1975, May 1, 1976, 
February 15, 1977, April 1, 1977, June 1, 1977, July 14, 1982, September 1, 
1986, April 13, 1987 and June 15, 1988, and Subordination Agreements dated 
December 29, 1950 and September 19, 1955 (said Original Indenture as so 
supplemented and amended being hereinafter collectively called the
"Indenture"), to which Indenture and all further instruments supplemental 
thereto reference is hereby made for a description of the properties mortgaged 
and pledged, the nature and extent of the security, the rights of the holders 
of said Bonds and the coupons appurtenant to coupon Bonds and of the Trustee 
and of the Company in respect of such security, and the terms and conditions 
upon which said Bonds are and are to be issued and secured.

            To the extent permitted by the Indenture and as provided therein,
with the consent of the Company and upon the written consent or affirmative vote
of at least sixty-six and two-thirds per cent in principal amount of the Bonds
then outstanding and entitled to consent, and of not less than sixty-six and
two-thirds per cent, in principal amount of the Bonds then outstanding and
entitled to consent of each series affected thereby in case one or more but less
than all of the series of Bonds issued under the Indenture are so affected, the
rights and obligations of the Company and of the holders of Bonds and coupons
appurtenant to coupon Bonds, and the terms and provisions of the Indenture and
of any instrument supplemental thereto may be modified from time to time,
provided that no such modification or alteration shall be made which would
postpone the date fixed herein or in the coupons or in the Indenture for the
payment of the principal of, or any installment of interest on, the Bonds, or
reduce the principal of, or the rate of interest payable on, the Bonds, or
reduce the percentage of the principal amount of Bonds the consent of which is
required for the authorization of any such modification or alteration, or which
would modify, without the written consent of the Trustee, the rights, duties or
immunities of the Trustee.

            As provided in the Indenture, said Bonds are issuable in series
which may vary as in the Indenture provided or permitted. This Bond is one of a
series of Bonds entitled "First Mortgage Bonds, Series Y, 9.49%, due June 15,
2018" (the "Series Y Bonds").

                                      -6-
                               
                           
<PAGE>
 
           Optional Redemption. On and after June 15, 1991, the Series Y Bonds
           -------------------
are subject to redemption, in whole or in part at the option of the Company upon
at least thirty (30) days and not more than fifty (50) days notice, mailed to
all registered owners of Series Y Bonds to be redeemed, at their respective
addresses as the same shall appear on the Bond register of the Company, all
subject to the conditions and as more fully provided in the Indenture, at the
following redemption price:


         
         
             A. On (but not before) or after June 15, 1991, and prior to June
                15, 2008, the price of the Series Y Bonds to be redeemed shall
                be (a) 100% of the principal amount of the Series Y Bonds to be
                redeemed, plus (b) accrued interest to the date fixed for
                redemption, plus (c) a Make-Whole Premium.
 
                The "Make-Whole Premium" shall mean the product of the excess,
                if any, of (a) the present value as of the date of redemption of
                all remaining scheduled principal and interest payments,
                including the principal payment at final maturity and the
                remaining scheduled interest payments on the Series Y Bonds
                (determined by discounting such amounts at the Reinvestment
                Yield from the respective dates on which such principal and
                interest payments are payable), minus 100% of the principal
                amount of the outstanding Series Y Bonds, times (b) a fraction,
                the numerator which is the principal amount of the Series Y
                Bonds being redeemed on such date pursuant to this Section and
                the denominator of which is 100% of the principal amount of the
                then outstanding Series Y Bonds.

                "Reinvestment Yield" shall mean the rate published in the weekly
                statistical release designated H.15(519) of the Federal Reserve
                System under the caption "U.S. Government Securities-Treasury
                Constant Maturities" the ("Statistical Release") (or if the
                Statistical Release is not published, such reasonably comparable
                index as may be designated by the holders of 66 2/3% in
                aggregate principal amount of the Series Y Bonds outstanding)
                for the maturity corresponding to the remaining Average Term to
                Maturity of the Series Y Bonds as of the date of redemption,
                rounded to the nearest month, plus 75 basis points. If no
                maturity exactly corresponds to such Average Term to Maturity of
                the Series Y Bonds, yields for the terms just before and just
                after the Average Term to Maturity of the Series Y Bonds shall
                be calculated pursuant to the immediately preceding sentence and
                the Reinvestment Yield will be interpolated from such yields on
                a straight-line basis, rounding each of the relevant periods to
                the nearest month. For the purposes of calculating the
                Reinvestment Yield, the most recent Statistical Release
                published prior to the date of determination hereunder shall be
                used.
                

                "Average Term to Maturity" shall mean, as of the time of
                determination thereof, the number of years obtained by dividing
                the Remaining Dollar-Years of the Series Y Bonds by the then
                outstanding principal amount of the Series Y Bonds. The term
                "Remaining Dollar-Years of the Series Y Bonds" shall mean the
                amount obtained by (1) multiplying the amount of each of the
                then remaining scheduled principal and interest payments,
                including the principal payments at final maturity, by the
                number of years (calculated to the nearest one-twelfth) which
                will elapse between the date of determination of the Average
                Term to Maturity of the Series Y Bonds and the date of each
                particular scheduled principal and interest payment and, (2)
                totaling all products obtained in (1).
 
                                      -7-
                                        
                                       
                                                                
<PAGE>
 
    B. On (but not before) June 15, 2008, or at any time thereafter, the Series
       Y Bonds are subject to redemption upon payment of accrued interest to the
       redemption date plus the applicable percentage of the principal amount
       thereof set forth below:
 
<TABLE>
<CAPTION>
          If Redeemed During 12
          Month Period Beginning
                June 15                              Percentage
          ----------------------                     ----------
          <S>                                        <C>
                  2008                                 102.945
                  2009                                 102.618
                  2010                                 102.291
                  2011                                 101.963
                  2012                                 101.636
                  2013                                 101.309
                  2014       .                         100.982
                  2015                                 100.654
                  2016                                 100.327
                  2017                                 100.000
</TABLE>       
 

           Mandatory sinking fund. The Series Y Bonds are subject to redemption,
           ----------------------
pursuant to the terms of the mandatory sinking fund provided for in the
Indenture. Beginning on June 15, 1998, and on each June 15 thereafter, there
shall become due and payable, and the Company will pay the principal sum of
$290,000 on the principal indebtedness evidenced by the Series Y Bonds plus
accrued interest thereon to the redemption date. No premium shall be payable in
connection with any payment made pursuant to this Section.

           Optional sinking fund. On June 15, 2008 and thereafter on any Series
           ---------------------
Y Sinking Fund Payment Date as defined in the Supplemental Indenture, the
Company shall have the option to double the mandatory sinking fund payment as
long as the aggregate principal amount of the Series Y Bonds retired pursuant to
this option does not exceed twenty-five per cent (25%) of the original principal
amount of the Series Y Bonds. The right to make such additional sinking fund
payments will not be cumulative and to the extent not exercised in any year will
lapse.

           The Series Y Bonds are further subject to redemption, in whole or in
part, by application of monies deposited with the Trustee in certain cases for
the release of properties from the lien of the Indenture, at any time with the
giving of the requested notice, all subject to the conditions and at the
redemption price as more fully set forth in the Indenture.

           The particular Series Y Bonds to be redeemed from time to time shall
be selected by the Trustee in the amount of $1,000 or an integral multiple
thereof and as nearly as practicable pro rata among the registered holders of
such Series Y Bonds according to the respective principal amounts of such Bonds.

           If this Bond or any portion thereof ($1,000 or a multiple) shall be
duly called for redemption as provided in the Indenture, this Bond or such
portion thereof shall (unless the Company shall default in the payment of the
redemption price) cease to bear interest from and after the date fixed for
redemption.
 
                                      -8-
                               
                                                         
<PAGE>
 
             Upon any partial redemption of this Bond, this Bond may, at the
option of the registered holder hereof, be either (a) surrendered to the Trustee
in exchange for one or more new Bonds of this Series for the principal amount of
the unredeemed portion of this Bond or (b) submitted to the Trustee for notation
hereon by the Trustee of the payment of the portion of the principal hereof so
called for redemption.

             If an Event of Default, as defined in the Indenture, shall occur,
the principal of this Bond may become or be declared due and payable, in the
manner and with the effect provided in the Indenture.

            This Bond is transferable by the registered owner hereof in person
or by attorney authorized in writing, at said principal corporate trust office
of the Trustee, upon surrender for cancellation of this Bond and on payment of
charges, and upon any such transfer a new Bond or Bonds, of the same series, for
the same aggregate principal amount, will be issued to the transferee in
exchange herefor.

            First Mortgage Bonds, Series Y, 9.49%, due June 15, 2018, are
issuable as fully registered Bonds without coupons of the denominations of
$1,000 and any multiple of $1,000 which may be executed by the Company and
delivered to the Trustee for authentication and delivery. The Series Y Bonds,
upon surrender thereof to the Trustee at its principal corporate trust office in
the Borough of Manhattan, The City of New York, are exchangeable for other Bonds
of the same series in such authorized denomination or denominations in the same
aggregate principal amount, as may be requested by the holders surrendering the
same.

            The Company and the Trustee may deem and treat the person in whose
name this Bond is registered as the absolute owner hereof, for the purpose of
receiving payment of or on account of the principal hereof and interest due
hereon, and neither the Company nor the Trustee shall be affected by any notice
to the contrary.

            No recourse shall be had for the payment of the principal of or the
interest on this Bond, or for any claim based hereon or otherwise in respect
hereof or of the Indenture or of any indenture supplemental thereto, against any
incorporator, stockholder, director or officer, as such, past, present or
future, of the Company or of any predecessor or successor corporation, either
directly or through the Company or any predecessor or successor corporation,
whether by virtue of any constitution, statute or rule of law, or by the
enforcement of any assessment or penalty or by any legal or equitable proceeding
or otherwise howsoever; all such liability being, by the acceptance hereof and
as a part of the consideration for the issuance hereof, expressly waived and
released by every holder hereof, as more fully provided in the Indenture;
provided, however, that nothing herein or in the Indenture contained shall be
taken to prevent recourse to and the enforcement of the liability, if any, of
any shareholder or any stockholder or subscriber to capital stock upon or in
respect of shares of capital stock not fully paid up.

            This Bond shall not be valid or become obligatory for any purpose
until the certificate of authentication hereon shall have been signed by the
Trustee, or its successor as Trustee, under the Indenture.

                                      -9-
  
<PAGE>
 
           IN WITNESS WHEREOF, the Company has caused this Bond to be signed in
its name by its President or one of its Vice-Presidents, and its corporate seal
to be impressed or imprinted hereon and attested by its Secretary or one of its
Assistant Secretaries.

         Dated:
         
                                       BLACK HILLS CORPORATION,
 
                                 
                                       By ______________________________________
                                                       President.
                                             
ATTEST:
 
 
__________________________________
           Secretary.
           
               (FORM OF TRUSTEE'S CERTIFICATE OF AUTHENTICATION)
         
            This is one of the Bonds, of the series designated therein,
described in the within mentioned Indenture.

                                       MANUFACTURERS HANOVER TRUST
                                         COMPANY, as Trustee,
                                 
                                       By _____________________________________
                                                   Authorized Officer.
                                          
            SECTION 1.03. The Series Y Bonds shall be redeemable (except as
otherwise provided in the last paragraph of this Section 1.03 or in Section 2.03
of this Supplemental Indenture) at the option of the Company, at any time and
from time to time, in whole or in part, on or after June 15, 1991, in the manner
and upon the notice provided in Article Ten of the Indenture, at the redemption
prices, and subject to the conditions, set forth in the form of Series Y Bonds
provided in Section 1.02 of this Supplemental Indenture, together, in each case,
with accrued interest to the redemption date. In the case of any redemption of
Series Y Bonds for which the Make-Whole Premium set forth in the form of Series
Y Bonds provided in Section 1.02 of this Supplemental Indenture may be payable,
the Company will give written notice to the registered owners of the Series Y
Bonds to be redeemed, and to the Trustee, by telecopy or other same day written
communication, three business days prior to the date fixed for redemption, which
notice shall set forth the Make-Whole Premium, if any, applicable to the Series
Y Bonds to be redeemed, and the calculations used to determine the amount of
such premium.

            Any monies applied to the redemption of Series Y Bonds pursuant to
the provisions of Section 8.08(a) of the Indenture on or after June 15, 1991 and
on or before June 14, 2008, and any monies applied to the redemption of Series Y
Bonds pursuant to the provisions of Section 8.08(b) of the the Indenture on or
before June 14, 2008, shall be so applied at a redemption price equal to 100% of
the principal amount of the Series Y Bonds to be redeemed, plus accrued interest
to the redemption date, plus an amount equal to the Make-Whole Premium set forth
in the form of the Series Y Bonds provided in Section 1.02
 
                                     -10-

                                
                                                      
<PAGE>
 
of this Supplemental Indenture. Monies applied to the redemption of Series Y
Bonds pursuant to the provisions of Section 8.08(a) or Section 8.08(b) of the
Indenture on or after June 15, 2008, shall be applied at a redemption price
equal to the applicable percentage of the principal amount of the Series Y Bonds
to be redeemed set forth in the form of the Series Y Bonds provided in Section
1.02 of this Supplemental Indenture plus accrued interest to the redemption
date.

             SECTION 1.04. Notwithstanding the provisions of Section 10.03 of
the Indenture, in case of the redemption at any time of less than all the
outstanding Series Y Bonds, the particular Bonds or parts thereof to be redeemed
shall be selected by the Trustee from the outstanding Series Y Bonds not
previously called for redemption as nearly as practicable pro-rata among the
registered holders of the Series Y Bonds according to the respective principal
amounts of such Bonds, provided that the portions of the principal of Series Y
Bonds at any time so selected for redemption in part shall be equal to $1,000 or
an integral multiple thereof.


                                  ARTICLE TWO
                        Sinking Fund for Series Y Bonds
                     
             SECTION 2.01. As a sinking fund for the retirement of Series Y
Bonds, so long as any of the Series Y Bonds shall be outstanding, the Company
will deposit with the Trustee on June 14, 1998, and annually thereafter on each
June 14 to and including June 14, 2017 (each such date being herein sometimes
referred to as a "Series Y Sinking Fund Payment Date"), cash in an amount
sufficient for the redemption of $290,000 aggregate principal amount of Series Y
Bonds on the next ensuing June 15 at a redemption price of 100% of the principal
amount thereof, together, in each case, with accrued interest to the redemption
date, and thereupon the Trustee shall apply such cash to the redemption of said
aggregate principal amount of the Bonds of said series on said next ensuing June
15. Any redemption of less than all of the Series Y Bonds shall not relieve the
Company of its obligation to redeem Series Y Bonds in accordance with the
requirements of this Section 2.01.

             SECTION 2.02. In addition to the mandatory sinking fund payments
required by Section 2.01 of this Supplemental Indenture, on June 15, 2008 and on
any Series Y Sinking Fund Payment Date thereafter, the Company shall have the
option to double the mandatory sinking fund payment as long as the aggregate
principal amount of the Series Y Bonds retired pursuant to this Section 2.02
does not exceed twenty-five percent (25%) of the original principal amount of
the Series Y Bonds. Any redemption pursuant to this Section 2.02 shall be at a
redemption price of 100% of the principal amount of the Series Y Bonds to be
redeemed, together in each case, with accrued interest to the redemption date.
Moneys deposited with the Trustee pursuant to this Section 2.02 shall be applied
by the Trustee to the redemption of Series Y Bonds on the next ensuing June 15.
In the event the Company shall elect to redeem Series Y Bonds pursuant to the
provisions of this Section 2.02, the Company shall give written notice of such
election to the Trustee on or before the 55th day prior to the applicable Series
Y Sinking Fund Payment Date.


            SECTION 2.03. Whenever the Trustee shall be required to redeem Bonds
pursuant to the provisions of Section 2.01 or Section 2.02 of this Supplemental
Indenture, the Trustee shall, on or before the 45th day prior to the Series Y
Sinking Fund Payment
 
                                     -11-

                                
                              
                                                    
<PAGE>
 
Date, proceed to select for redemption, from the Bonds of said series, in the
manner provided in Section 1.04 of this Supplemental Indenture, the aggregate
principal amount of Bonds of said series required by the provisions of said
Section 2.01 and Section 2.02 to be redeemed by application of the cash to be
paid to the Trustee on said Series Y Sinking Fund Payment Date, and for and on
behalf of the Company and in the name of the Company, the Trustee shall give
notice, as required by the provisions of Article Ten of the Indenture, of the
redemption for the Series Y Sinking Fund of the Bonds so selected. Subject to
the provisions of this Article, the redemption of such Bonds shall be effected
in the manner and upon the terms provided in Section 10.03 of the Indenture at
the sinking fund redemption price of 100% of the principal amount thereof,
together, in each case, with accrued interest to the redemption date.
 
                                 ARTICLE THREE
 
                           Miscellaneous Provisions
 
             SECTION 3.01. The Company covenants that so long as any Series Y
Bonds shall remain outstanding it will comply with the covenants contained in
Sections 9.06, 9.15, 9.16, 9.20 and 9.22 of the Indenture.
 
             SECTION 3.02 The Company further covenants that so long as any
Series Y Bonds shall remain outstanding, the Company will not, without the
consent of the holder of each of the Series Y Bonds, revise the original
schedule of sinking fund payments as provided in Section 2.01 of this
Supplemental Indenture or modify any of the redemption prices for the Series Y
Bonds as provided in the form of the Series Y Bonds provided in Section 1.02 of
this Supplemental Indenture. The provisions of this covenant can only be
modified, amended or otherwise waived with approval from all of the holders of
Series Y Bonds outstanding, excluding any Series Y Bonds held by the Company.
 
             SECTION 3.03. The aggregate principal amount of Bonds which,
immediately after the authentication and delivery of the Series Y Bonds to be
issued under this Supplemental Indenture, will be outstanding under the
provisions of, and secured by, the Indenture, as amended by this Supplemental
Indenture, will be $36,484,000, consisting of the Bonds of Series I through
Series O and Series R through Series X hereinbefore set forth in the second
recital of this Supplemental Indenture and $6,000,000 aggregate principal amount
of Series Y Bonds hereby created.
 
             SECTION 3.04. The Company, by the execution hereof, acknowledges
that a true copy of this Supplemental Indenture has been delivered to and
received by it.
 
             SECTION 3.05. Except as amended by this Supplemental Indenture, all
the provisions, terms and conditions of the Indenture shall continue in full
force and effect.
 
             SECTION 3.06. This Supplemental Indenture may be executed in
several counterparts, all or any of which may be treated for all purposes as one
original and shall constitute and be one and the same instrument.
 
                                     -12-

                                                            


                                                           


                                                           



                                                     
<PAGE>
 
            IN WITNESS WHEREOF, BLACK HILLS CORPORATION, party hereto of the
first part, has caused this Supplemental Indenture to be executed on its behalf
by its Chairman of the Board or its President or one of its Vice Presidents and
its corporate seal to be hereto affixed and to be attested by its Secretary or
an Assistant Secretary, and MANUFACTURERS HANOVER TRUST COMPANY, party hereto of
the second part, in evidence of its acceptance of the trust hereby created, has
caused this Supplemental Indenture to be executed on its behalf by one of its
Vice Presidents or Assistant Vice Presidents and its corporate seal to be hereto
affixed and to be attested by its Secretary or an Assistant Secretary, all as of
the day and year first above written.

                                       BLACK HILLS CORPORATION
 
[Corporate Seal]
 
                                       By            George T. Locke
                                          ------------------------------------- 
                                               Senior Vice President-Finance

ATTEST:

         David E. Morrill
- --------------------------------------
        Assistant Secretary
       
Signed, sealed and delivered by
 BLACK HILLS CORPORATION in the
 presence of:
 
            Neil Farina
- --------------------------------------
 

         Patricia McDermott
- --------------------------------------
    
                                       MANUFACTURERS HANOVER TRUST
                                                 COMPANY
[Corporate Seal]
                                       By            W. B. Dodge
                                          -------------------------------------
                                               Assistant Vice President
            

ATTEST:

          Mark G. Walsh
- --------------------------------------
        Assistant Secretary
 
Signed, sealed and delivered by
 MANUFACTURERS HANOVER TRUST
 COMPANY in the presence of:
 
            Neil Farina
- --------------------------------------
 

         Patricia McDermott
- --------------------------------------
 
                                     -13-
                              
<PAGE>
 
STATE OF NEW YORK     )
                      ) ss.:
COUNTY OF NEW YORK    )
 
         On this 15th day of June, 1988 before me, Peter Ferreri, the
                 ----------------------            -------------
undersigned officer, personally appeared George T. Locke, to me personally
known, who acknowledged himself to be, and being by me duly sworn, did say that
he is Senior Vice President - Finance of BLACK HILLS CORPORATION, a corporation,
and that the seal affixed to the foregoing instrument is the corporate seal of
said corporation and that said instrument was executed by, and signed in the
name of, the corporation, by him, as such Senior Vice President - Finance and
sealed in behalf of the corporation by authority of its Board of Directors for
the purposes therein contained, and the said George T. Locke acknowledged the
same as the free act and deed of said corporation.

         IN WITNESS WHEREOF, I hereunto set my hand and official seal.
         
[Notarial Seal]                                    Peter Ferreri
                                       -----------------------------------------
                                                   Notary Public
                                                   Peter Ferreri
                                           Notary Public, State of New York
                                                   No. 41-6278425
                                              Qualified in Queens County
                                        Certificate filed in New York County
                                        Commission Expires October 31, 1988
                                 
                                     -14-

                                 
                                                                     
<PAGE>
 
STATE OF NEW YORK     )
                      ) ss.:
COUNTY OF NEW YORK    )


            On this 15th day of June, 1988 before me, Peter Ferreri, the  
                    ----------------------            -------------
undersigned officer, personally appeared W. B. Dodge, to me personally known,
                                         -----------
who acknowledged himself to be, and being by me duly sworn, did say that he is
Assistant Vice President of MANUFACTURERS HANOVER TRUST COMPANY, a corporation,
and that the seal affixed to the foregoing instrument is the corporate seal of
said corporation and that said instrument was executed by, and signed in the
name of, the corporation, by him, as such Assistant Vice President, and
sealed in behalf of the corporation by authority of its Board of Directors for
the purposes therein contained, and the said W. B. Dodge acknowledged the same
                                             -----------
as the free act and deed of said corporation.

         IN WITNESS WHEREOF, I hereunto set my hand and official seal.
         
[Notarial Seal]                                    Peter Ferreri
                                       -----------------------------------------
                                                   Notary Public
                                                   Peter Ferreri
                                           Notary Public, State of New York
                                                   No. 41-6278425
                                              Qualified in Queens County
                                        Certificate filed in New York County
                                        Commission Expires October 31, 1988


            
                                     -15-

<PAGE>
 
                                                                 EXHIBIT 4.06
                                                                 ------------

                                    PROPOSED
                                    --------

                      TWENTY EIGHTH SUPPLEMENTAL INDENTURE

     TWENTY EIGHTH SUPPLEMENTAL INDENTURE, dated as of the ______ day of
_________________, 19__, between Black Hills Corporation (formerly named and now
operating its electric utility division as Black Hills Power and Light Company),
a corporation organized and existing under the laws of the State of South Dakota
(hereinafter called the "Company"), party of the first part, and Chemical Bank,
a corporation organized and existing under the laws of the State of New York, as
Trustee under the Indenture hereinafter mentioned (hereinafter called the
"Trustee"), party of the second part.

     WHEREAS, in order to secure an authorized issue of First Mortgage Bonds of
the Company, the Company has executed and delivered an Indenture of Mortgage and
Deed of Trust to Central Hanover Bank and Trust Company (subsequently known as
The Hanover Bank) as Trustee, dated September 1, 1941, hereinafter referred to
as the "Original Indenture," and has also executed and delivered to said Trustee
and to Manufacturers Hanover Trust Company (which on September 8, 1961 became
the Trustee under the Original Indenture, as theretofore supplemented and
amended, by virtue of the merger of said The Hanover Bank into Manufacturers
Trust Company, under said name Manufacturers Hanover Trust Company), as Trustee,
various Supplemental Indentures supplementing and/or modifying the Original
Indenture, respectively referred to herein and dated as follows:
 
<TABLE>
<S>                                       <C>
   First Supplemental Indenture             July 15, 1945
   Second Supplemental Indenture          January 15, 1948
   Third Supplemental Indenture           January 15, 1949
   Fourth Supplemental Indenture            March 1, 1950
   Fifth Supplemental Indenture             March 1, 1952
   Sixth Supplemental Indenture             July 1, 1956
   Seventh Supplemental Indenture            May 1, 1957
   Eighth Supplemental Indenture             May 1, 1959
   Ninth Supplemental Indenture             April 1, 1960
   Tenth Supplemental Indenture            August 1, 1960
   Eleventh Supplemental Indenture          June 1, 1961
   Twelfth Supplemental Indenture          October 1, 1962
   Thirteenth Supplemental Indenture         May 1, 1963
   Fourteenth Supplemental Indenture        June 1, 1969
   Fifteenth Supplemental Indenture         June 15, 1974
   Sixteenth Supplemental Indenture        August 1, 1974
   Seventeenth Supplemental Indenture       July 15, 1975
   Eighteenth Supplemental Indenture         May 1, 1976
   Nineteenth Supplemental Indenture      February 15, 1977
   Twentieth Supplemental Indenture         April 1, 1977
   Twenty First Supplemental Indenture      June 1, 1977
</TABLE>
<PAGE>
 
<TABLE>
<S>                                         <C>
   Twenty Second Supplemental Indenture       July 14, 1982
   Twenty Third Supplemental Indenture      September 1, 1986
   Twenty Fourth Supplemental Indenture     April 13, 1987
   Twenty Fifth Supplemental Indenture        June 15, 1988
   Twenty Sixth Supplemental Indenture        May 15, 1991
   Twenty Seventh Supplemental Indenture      June 1, 1991
</TABLE>

     [list any additional Supplements existing at the time of execution]

which, as supplemented and amended by said ___________________________ [the
number of supplements at time of execution] Supplemental Indentures is
hereinafter referred to as the "Indenture"; and

     WHEREAS, Chemical Bank is the successor by merger with Manufacturers
Hanover Trust Company and now is the Trustee under the Indenture; and

     WHEREAS, pursuant to the provisions of the Indenture, First Mortgage Bonds
have been duly issued and are presently outstanding and secured by the Indenture
as follows:

                                              Principal Amount
                    Series                      Outstanding
                    ------                      -----------

     [List all Series and principal amount outstanding at time of execution.]

     WHEREAS, the Company deems it advisable that the Indenture be amended as
herein provided; and

     WHEREAS, the Company since May 1, 1994 has issued Bonds (referred to herein
as "Consent Bonds") under the Indenture pursuant to Supplemental Indentures that
each contained a consent provision providing that the Holders and any successor
Holders of such Bonds shall be deemed to have consented to the execution and
adoption of this Twenty Eighth Supplemental Indenture, and authorizing the
Trustee to receive and accept such provision as a consent to execute this
supplement under the provisions of Section 18.11 of the Indenture in lieu of the
holding of a meeting of Bondholders; and

     WHEREAS, the total of (i) the principal amount of all Consent Bonds
outstanding and (ii) the principal amount of all Bonds outstanding issued prior
to May 1, 1994, the Holders of which have consented in writing to the adoption
of this Twenty Eighth Supplemental Indenture, constitutes the consent of the
requisite amount of Bonds outstanding under Section 18.11 of the Indenture to
authorize the execution thereof; and

                                       2
<PAGE>
 
     WHEREAS, the Company, in the exercise of the powers and authority conferred
upon and reserved to it under and by virtue of the provisions of the Indenture,
and particularly the provisions contained in Articles Seventeen and Eighteen
thereof, and pursuant to appropriate resolutions of its Board of Directors has
duly resolved and determined to make, execute and deliver to the Trustee the
Twenty Eighth Supplemental Indenture in the form hereof for the purposes therein
provided; and

     WHEREAS, all conditions and requirements necessary to make this Twenty
Eighth Supplemental Indenture a valid, binding and legal instrument in
accordance with its terms have been done, performed and fulfilled, and the
execution and delivery hereof have been in all respects duly authorized.

     NOW, THEREFORE, THIS INDENTURE WITNESSETH:

     That the Company, in consideration of the premises and one dollar to it
duly paid by the Trustee at or before the ensealing and delivery of these
presents, the receipt whereof is hereby acknowledged, and of other good and
valuable consideration, in order to amend certain provisions of the Indenture,
hereby further covenants and agrees to and with the Trustee and its successors
in the trust under the Indenture for the benefit of all those who shall from
time to time hold the Bonds as follows:


                                  ARTICLE ONE

                          Amendments to the Indenture

     1.01.  Deletion of Contracts, Leases and Franchises from the Lien of the
            -----------------------------------------------------------------
Indenture.  The following granting clauses:  Eleventh--Certain Contracts,
- ---------                                                                
Twelfth--Leases and Thirteenth--Franchises are deleted from the Indenture and
any and all property interests described in the Eleventh, Twelfth and Thirteenth
Granting Clauses are hereby released from the Lien of the Indenture.
Subparagraphs (e) and (f) of Section 7.01 of Article Seven are deleted from the
Indenture.


     1.02.  Obligation to Maintain Business.  Section 9.13 of Article Nine of
            -------------------------------                                  
the Indenture is amended so as to read as follows:

          SECTION 9.13.  Except in the case of a merger, consolidation,
     conveyance or transfer as in Article Fourteen provided, the Company will at
     all times maintain its corporate existence and right to carry on business,
     and will duly procure all renewals and extensions thereof, and shall do or
     cause to be done all things necessary to preserve and keep in full force
     and effect its

                                       3
<PAGE>
 
     corporate existence and rights and franchises of the Company; provided,
     however, that the Company shall not be required to preserve any such right
     or franchise if, in the judgment of the Company, the preservation thereof
     is no longer desirable in the conduct of the business of the Company and
     the loss thereof would not adversely affect the interests of the
     Bondholders in any material respect.  The Company will not enter into any
     merger or consolidation, or make any conveyance or lease of all or
     substantially all of the Trust Estate as an entirety unless, in connection
     therewith, the Company and/or the successor corporation and/or the lessee,
     as the case may be, shall observe and comply with the terms and conditions
     of Article Fourteen applicable to such transaction.

     The first paragraph of Section 9.05 of Article Nine of the Indenture is
amended so as to read as follows:

          SECTION 9.05.  The Company covenants that the business of the Company
     will be carried on and conducted in an efficient manner; all property,
     plants, appliances and equipment of the Company useful in the carrying on
     of its business will be kept in repair and maintained in good working order
     and condition, and if worn or injured will be replaced by other property
     suitable to the business of the Company and of at least equal value.


     1.03.  Excepted Property.  The Sixteenth Granting Clause of the Indenture
            -----------------                                                 
is hereby amended so as to read as follows:

                                   SIXTEENTH

                               EXCEPTED PROPERTY

     There is, however, expressly excepted and excluded from the Lien of this
Indenture the following described property of the Company, herein sometimes
referred to as "Excepted Property":

          A.  all cash on hand, in banks or in other financial institutions with
     which the Company maintains deposits, shares of stock, bonds, notes,
     evidences of indebtedness and other securities not hereafter paid or
     delivered to, deposited with, or held by, the Trustee hereunder or required
     so to be;

          B.  all contracts, leases and other agreements of whatsoever kind and
     nature (including pole attachment agreements and joint pole agreements),
     contract rights, bills, notes and other instruments, accounts receivable,
     claims, credits,

                                       4
<PAGE>
 
     demands, judgments, choses in action, patents, patent licenses and other
     patent rights, patent applications, trade names, trademarks and other
     general intangibles;

          C.  all permits, licenses, franchises (including municipal franchises
     and other rights to use public ways) and rights (however characterized)
     granted by any governmental entity with respect to air, water or other
     types of pollution or pollution credits;

          D.  all motor vehicles, automobiles, buses, trucks, truck cranes,
     tractors, trailers and similar vehicles, movable equipment, all rolling
     stock, railcars, containers and other railroad equipment, all vessels,
     boats, barges and other marine equipment, all airplanes, airplane engines
     and flight equipment, and all components, spare parts, accessories,
     supplies and fuel used or to be used in connection with any of the
     foregoing;

          E.  all goods, wares, merchandise, equipment, spare parts and tools
     held for sale or lease in the ordinary course of business or for use or
     consumption in, or in the operation of, any properties of, or for the
     benefit of, the Company, or held in advance of use thereof for maintenance,
     replacement or fixed capital purposes; all fuel, materials and supplies and
     other personal property which are consumable (otherwise than by ordinary
     wear and tear) in their use in the electric utility business;

          F.  all office furniture and office equipment; all satellites and
     other equipment and materials used or to be used in outer space; all
     business machines; all communications equipment (including telephone
     equipment); all computer equipment; all record production, storage and
     retrieval equipment; and all components, spare parts, accessories, programs
     (other than computer software) and supplies used or to be used in
     connection with any of the foregoing;

          G.  all crops, timber, sand, gravel, rocks, earth, natural gas, coal,
     ore, uranium, gas, oil and other minerals harvested, mined or extracted or
     otherwise separated from the land, or lying or being upon, within or under
     any properties of the Company, including the Trust Estate, all mineral
     rights, leases and royalties and income therefrom, and all rights to
     explore for minerals, and gas or oil wells or any lease or real estate
     acquired for the purpose of obtaining gas or oil rights;

          H.  all electric energy, steam, water, ice and other products
     generated, manufactured, produced, provided or purchased by the Company for
     sale, transmission or distribution or used or to be used by the Company;

          I.  all leasehold interests and leasehold improvements;

                                       5
<PAGE>
 
          J. all property, real, personal and mixed, which is:

               (i)  not specifically subjected or required to be subjected to
          the Lien of this Indenture by any express provision hereof; and

               (ii) not used or to be used in the electric utility business, or
          in connection with the operation of any property specifically
          subjected or required to be subjected to the Lien of this Indenture by
          the express provisions hereof;

          K.  the Company's franchise to be a corporation; and

          L.  all books and records;

     it being understood that the Company may, however, pursuant to the
     Fifteenth Granting Clause of the Indenture, subject to the Lien of this
     Indenture any Excepted Property, whereupon the same shall cease to be
     Excepted Property.


     1.04.  Property Additions--Definition.  Paragraph A of Section 4.01 of
            ------------------------------                                 
Article Four of the Indenture is hereby amended so as to read as follows:

          A.  The term "Property Additions" shall mean real estate owned in fee,
     easements and rights of way in respect of real estate, buildings, electric
     lines, reservoirs, structures, machinery, meters, equipment and other
     tangible properties, real, personal or mixed useful to the Company in the
     business (hereinafter called "Electric Business") of generation,
     transmission, distribution and/or sale of electricity, including whole or
     undivided interests in any of such properties (such properties being
     hereinafter sometimes referred to as "Electric Properties") purchased,
     constructed or otherwise acquired by the Company subsequent to October 31,
     1941; and the term "Property Additions" shall include

               (1)  property of the character above described acquired by the
          Company by merger or consolidation as well as property purchased or
          constructed by the Company;

               (2)  new plants and systems of the character above described;

               (3)  all construction work in progress in the amount as recorded
          on the books of account of the Company under generally accepted
          accounting principles;

                                       6
<PAGE>
 
               (4)  property of the character above described constructed or
          acquired to replace an item of property whose retirement has been
          credited to plant account; and

               (5)  any Excepted Property and other property of the Company that
          the Company elects to be included under the Lien of the Indenture.

     If the Company shall, as provided in Article Fourteen, consolidate with or
     merge into or convey all or substantially all of the Trust Estate as an
     entirety to any other corporation, and such successor corporation shall
     execute a supplemental indenture of the character described in Paragraph A
     of Section 14.02, all property of the character herein described as
     Property Additions and owned by such successor corporation at the time of
     such consolidation, merger or conveyance, or acquired by it by such
     consolidation, merger or conveyance (excluding Bonded Property acquired
     from the Company), shall be deemed to be Property Additions acquired by
     such successor corporation at the date upon which it became such successor
     corporation.

          Among other properties not constituting Property Additions under the
     foregoing provisions, the term "Property Additions" shall not be deemed to
     include

               (7)  any item of property constructed or acquired to replace a
          similar item of property whose retirement has not been credited to
          plant account; or any property whose cost has been charged, or is
          properly chargeable, to repairs or maintenance or other operating
          expense account, or whose cost has not been charged, or is not
          properly chargeable, to plant account;

               (8)  any Excepted Property unless the Company elects to cause the
          Excepted Property to be subject to the Lien of the Indenture; or

               (9)  going concern value or good will, or franchises or
          governmental permits granted to or acquired by the Company, separate
          and distinct from the property operated thereunder.


     1.05.  Additions Credit not Bonded Property.  Clause (2) of Paragraph K of
            ------------------------------------                               
Section 4.01 of Article Four is hereby amended so as to read as follows:

                                       7
<PAGE>
 
          (2)  All Property Additions which have been made the basis for the
     authentication and delivery of Bonds or the release of any Bonded Property
     from the Lien of this Indenture or the withdrawal of any Bonded Cash (or
     Unbonded Cash, if withdrawn under Section 8.03) from the Trustee, but not
     including Additions Credits within the meaning of Paragraph N of this
     Section.


     1.06.  Addition Credit--Definition.  Paragraph N of Section 4.01 of Article
            ---------------------------                                         
Four of the Indenture is hereby amended so as to read as follows:

          N.  "Additions Credit."

               If and whenever any Net Bondable Additions or Gross Bondable
          Additions are certified or made a part of an application to the
          Trustee for any purpose under this Indenture, any amount is in excess
          of that required for such purpose, the excess amount shall constitute
          an Additions Credit and may be carried forward and used as Property
          Additions for additional certifications and applications under this
          Indenture.  The Company shall have the right, at any time and from
          time to time, to establish an Additions Credit by delivering to the
          Trustee the Certificates, Opinions and Other Instruments which would
          be required to be delivered to the Trustee under Section 4.02 B,
          Clauses (1) through (11), (13) and (14) and Sections 4.02, Paragraphs
          C, D and E of this Indenture.


     1.07.  Certifiable Net Earnings.  Paragraph O of Section 4.01 of Article
            ------------------------                                         
Four of the Indenture as amended by Section 3.04 of Article Three of the Third
Supplemental Indenture, and as amended by Section 1.03 of Article One of the
Nineteenth Supplemental Indenture is hereby further amended so as to read as
follows:

          O.  The "Certifiable Net Earnings" of the Company for any particular
     period shall be computed and ascertained by deducting from the total of the
     Gross Operating Revenues of the Company for such period the following:

               All operating expenses and other proper charges (other than those
          charged to capital accounts or surplus) including (a) all Federal,
          state and local taxes (other than taxes in respect of income or
          profits and other taxes imposed on or measured by income or profits);
          and (b) rentals, insurance, current repairs and maintenance; but
          excluding (i) provisions for reserves for renewals, replacements,
          depreciation, depletion or retirement of property (or any expenditures
          therefor), or

                                       8
<PAGE>
 
          provisions for amortization of property, (ii) expenses or provisions
          for interest on any indebtedness of the Company, for the amortization
          of debt discount, premium, expense or loss on reacquired debt, for any
          maintenance and replacement, improvement or sinking fund or other
          device for the retirement of any indebtedness, or for other
          amortization, (iii) expenses or provisions for any nonrecurring charge
          to income or to retained earnings of whatever kind or nature
          (including without limitation the recognition of expense or impairment
          due to the nonrecoverability of assets or expense), whether or not
          recorded as a nonrecurring charge in the Company's books of account,
          and (iv) provisions for any refund of revenues previously collected or
          accrued by the Company subject to possible refund.

          The Gross Operating Revenues of the Company shall consist of Gross
     Utility Operating Revenues of the Company, plus the Net Non-Operating
     Income of the Company.  The term "Gross Utility Operating Revenues" of the
     Company shall mean the aggregate gross operating revenues derived from the
     operation of the utility properties owned or leased by the Company.  The
     term "Net Non-Operating Income" of the Company shall mean net income
     derived from but not necessarily limited to the following:  (a)
     merchandising, jobbing and contract work; (b) rental of non-utility
     properties; (c) interest and dividend income including dividends from
     Subsidiaries; (d) allowance for funds used during construction; and (e)
     other miscellaneous non-operating income; provided, however, that profits
     or losses resulting from the sale, abandonment or other disposition of
     capital assets or securities of the Company and the Company's equity in the
     undistributed earnings of Subsidiaries, shall not be taken into account in
     the calculation of Net Non-Operating Income.

          Subject to the foregoing provisions of this Section, all
     determinations of earnings pursuant to this Indenture shall be made, and
     all balance sheets and other financial statements to be delivered hereunder
     shall be prepared, in accordance with the practice prescribed by any
     regulatory authority having jurisdiction over the Company or other lawfully
     prescribed practice or, in the absence of any practice prescribed by law,
     in accordance with sound accounting practice and, where consistent with
     such practice and with the foregoing provisions of this Section, on the
     same basis as that used in preparing the financial statements included in
     the annual report of the Company for the preceding fiscal year.

                                       9
<PAGE>
 
     1.08.  Property Additions Certificate--Amendment to Conform to Other
            -------------------------------------------------------------
Provisions.  Clauses (10) and (12) of Paragraph B of Section 4.02 of Article
- ----------                                                                  
Four of the Indenture is hereby amended so as to read as follows:

          (10)  That no part of the Property Additions described in said
     Certificate is property the construction or acquisition of which under the
     provisions of Section 4.01 is not permitted to be made the basis of the
     authentication and delivery of Bonds under this Article.

          (12)  Whether there is any unused Additions Credit which the Company
     desires to use, in whole or in part, as a basis for the authentication and
     delivery of the Bonds then applied for, and if so, a statement of the
     entire amount which the Company so desires to use, of each such unused
     Additions Credit.


     1.09.  Earnings-to-Interest Coverage Requirements.  Paragraph F of Section
            ------------------------------------------                         
4.02 of Article Four of the Indenture as previously amended by Section 1.04 of
the Nineteenth Supplemental Indenture is hereby amended so as to read as
follows:

          F.  A Net Earnings Certificate of the Company, complying with the
     provisions of Section 1.02, dated not more than 45 days prior to the
     application for the authentication and delivery of such Bonds, certified by
     an Accountant, and setting forth:

               (1)  The amount of the Certifiable Net Earnings of the Company,
          for a period of 12 consecutive calendar months within the 18 calendar
          months immediately preceding the date on which the application for the
          authentication and delivery of the Bonds is made, and stating
          separately the Gross Utility Operating Revenues and the Net Non-
          Operating Income and the operating expenses of the Company and other
          deductions from such Gross Utility Operating Revenues and Net Non-
          Operating Income pursuant to Paragraph O of Section 4.01, with the
          principal subdivisions thereof.

               (2)  The aggregate amount of the annual "Interest Charges on
          Bonds and Prior Lien Debt" of the Company, which term shall mean the
          annual interest charges on

                    (a)  all Bonds outstanding hereunder at the date of said
               Certificate, provided, however, that in the case of any Bonds
               which shall at such time be pledged as security for any

                                       10
<PAGE>
 
               indebtedness of the Company, the amount of the annual interest
               charges on such pledged Bonds shall be deemed to be either the
               amount of the annual interest charges on such indebtedness or the
               amount of the annual interest charges on such pledged Bonds,
               whichever shall be greater; and

                    (b)  all Bonds the authentication and delivery of which is
               applied for in such application and in any other pending
               application; and

                    (c)  all indebtedness secured by a lien upon the Trust
               Estate, or any part thereof, prior to the lien of this Indenture,
               other than a Prepaid Lien;

          provided, however, that there shall be excluded from such computation
          the annual interest charges on any Bonds or indebtedness which is to
          be paid, redeemed or otherwise retired, or provision for the
          retirement of which is to be made, so that the same will cease to be
          outstanding prior to or concurrently with the authentication and
          delivery of the Bonds then applied for.

               (3)  That the amount of the Certifiable Net Earnings of the
          Company set forth as provided by Clause (1) of this Paragraph have
          been at least equal to two (2) times the aggregate amount of the
          annual Interest Charges on Bonds and Prior Lien Debt of the Company as
          provided by Clause (2) of this Paragraph.

               (4)  That such Certifiable Net Earnings have been computed and
          ascertained as provided in Paragraph O of Section 4.01.

          If the annual Interest Charges on Bonds and Prior Lien Debt shall be
     increased after the date of the Earnings Certificate hereinabove in this
     Paragraph described, and before the authentication and delivery of the
     Bonds then applied for, the Company will file with the Trustee a new
     Earnings Certificate showing the amount of said annual Interest Charges on
     Bonds and Prior Liens as so increased--it being the intention hereof that
     no Bonds shall be authenticated and delivered under the provisions of this
     Article, unless the ratio provided for by Clause (3) of this Paragraph
     shall have been established with respect to the aggregate amount of the
     annual Interest Charges on Bonds and Prior Liens of the Company as
     constituted at the time of the authentication and delivery of the Bonds
     then applied for; but the Trustee shall, subject to the provisions of
     Section 15.02, be entitled to assume, in the

                                       11
<PAGE>
 
     absence of such new Earnings Certificate, that the aggregate amount of the
     annual Interest Charges on Bonds and Prior Lien Debt of the Company, as
     constituted at the time of the authentication and delivery of the Bonds
     then applied for, are as stated in the Earnings Certificate filed with the
     Trustee as aforesaid.

          The Earnings Certificate provided for in this Paragraph shall be
     certified by an Independent Public Accountant selected by the Company and
     approved by the Trustee, in the exercise of reasonable care, if, but only
     if, the aggregate principal amount of the Bonds to be authenticated and
     delivered on the basis thereof and of other Bonds authenticated and
     delivered since the commencement of the then current calendar year (other
     than those with respect to which an Earnings Certificate is not required or
     with respect to which an Earnings Certificate verified by an Independent
     Public Accountant has previously been furnished) is 10% or more of the
     aggregate principal amount of the Bonds at the time outstanding.


     1.10.  Summary Certificate.  Paragraph G of Section 4.02 of Article Four of
            -------------------                                                 
the Indenture is hereby amended so as to read as follows:

          G.  A summary certificate and computation of the Company complying
     with the provisions of Section 1.02, determining the Net Bondable Additions
     in conformity with the provisions of this Indenture as amended.


     1.11.  Amount of Property Additions Required for New Bonds.  Section 4.03
            ---------------------------------------------------               
of Article Four of the Indenture is hereby amended so as to read as follows:

          SECTION 4.03.  Upon compliance with the provisions of Section 4.02,
     the Trustee shall authenticate and deliver Bonds in an aggregate principal
     amount up to, but not exceeding seventy percent (70%) of the amount of Net
     Bondable Additions shown by the summary certificate and computation filed
     pursuant to Paragraph G of Section 4.02.


     1.12.  Additional Authorization to Issue Bonds.  The Indenture is amended
            ---------------------------------------                           
by the addition of the following Section 4.04 to Article Four of the Indenture.

          SECTION 4.04.  Notwithstanding anything in this Indenture to the
     contrary, upon the Written Order of the Company, the Trustee shall

                                       12
<PAGE>
 
     authenticate and deliver Bonds in an aggregate principal amount up to, but
     not exceeding seventy percent (70%) of

               A.  the total amount of Property Additions which the Company
          certifies to the Trustee as of May 1, 1994 and as of each May 1
          thereafter to meet the requirements of Section 9.06 until such time
          that Section 9.06 is deleted by the execution of the Twenty Eighth
          Supplemental Indenture; and

               B.  fourteen and two-tenths percent (14.2%) of the total amount
          of all Property Additions certified to the Trustee and used as
          Bondable Additions for the issuance of Bonds during the period
          beginning May 1, 1994 and ending on the execution date of the Twenty
          Eighth Supplemental Indenture.

     The Company shall furnish the Trustee a Certificate of the Company
referencing and documenting the amount of Property Additions previously
certified and used as provided at Paragraphs A and B of this Section, a summary
certificate and computation determining the amount of Bonds that may be issued
under this Section 4.04 and an Opinion of Counsel as required by Paragraph I of
Section 4.02; provided that subparagraph (1) thereunder shall refer to the basis
of this Section 4.04 rather than Net Bondable Additions and subparagraphs (2),
(3), (4) and (5) are not to be included in such opinion.


     1.13.  Release of Property from Indenture.  Section 7.02 of Article Seven
            ----------------------------------                                
of the Indenture is hereby amended so as to read as follows:


          SECTION 7.02.

               A.  Definition of "Fair Value."  For the purposes of this Section
                   -------------------------                                    
          7.02, "Fair Value" when applied to property is its value as determined
          without deduction for any Prior Liens upon such property and without
          deduction to reflect that such property may be of value only to the
          Company or another operator of the Trust Estate as a whole, which
          value may be determined without physical inspection by use of
          accounting and engineering records and other data maintained by, or
          available to, the Company.


               B.  Release Based on Bond Ratio.  Unless an Event of Default
                   ---------------------------                             
          shall have occurred and be continuing, upon receipt of a

                                       13
<PAGE>
 
          Written Order of the Company requesting the release of any of the
          Trust Estate pursuant to this Paragraph B, the Trustee shall execute
          and deliver to the Company the documents and instruments described in
          Paragraph B, releasing from the Lien of this Indenture any of the
          Trust Estate if the Fair Value of all of the Trust Estate (excluding
          the Trust Estate to be released but including any Trust Estate to be
          acquired by the Company with the proceeds of, or otherwise in
          connection with, such release) stated on the Engineer's certificates
          delivered pursuant to Clause 2 of Paragraph B and Clause (3) of
          Paragraph B, equals or exceeds an amount equal to twenty-fourteenths
          (20/14) of the aggregate principal amount of Bonds outstanding at the
          date of such Written Order of the Company as stated on the Certificate
          of the Company delivered pursuant to Clause (4) of Paragraph B, upon
          receipt by the Trustee of:

                    (1)  appropriate documents and instruments releasing without
               recourse the interest of the Trustee in the Trust Estate to be
               released, and describing in reasonable detail the Trust Estate to
               be released;

                    (2)  an Engineer's certificate, dated the date of such
               Written Order of the Company, stating (i) that the signers of
               such Engineer's certificate have examined the Certificate of the
               Company delivered pursuant to Clause (4) of Paragraph B in
               connection with such release, (ii) the Fair Value, in the opinion
               of the signer of such Engineer's certificate, of (A) all of the
               Trust Estate, and (B) the Trust Estate to be released, in each
               case as of a date not more than 90 days prior to the date of such
               Written Order of the Company, and (iii) that in the judgment of
               such signers, such release (A) will not materially adversely
               affect the Company's Electric Business, and (B) will not impair
               the security under this Indenture in contravention of the
               provisions hereof;

                    (3)  in case any Property Additions are being acquired by
               the Company with the proceeds of, or otherwise in connection
               with, such release, an Engineer's certificate, dated the date of
               such Written Order of the Company, as to the Fair Value, as of a
               date not more than 90 days prior to the date of such Written
               Order of the Company, of the Property Additions being so acquired
               (and if within six months prior to the date of acquisition by the
               Company of the Property Additions being so

                                       14
<PAGE>
 
               acquired, any property included within such Property Additions
               had been used or operated by others than the Company in a
               business similar to that in which it has been or is to be used or
               operated by the Company, and the Fair Value thereof to the
               Company, as set forth in such Engineer's certificate, is not less
               than one percent (1%) of the aggregate principal amount of Bonds
               then outstanding, such certificate shall be an Independent
               Engineer's Certificate);

                    (4)  a Certificate of the Company, dated the date of such
               Written Order of the Company, stating (i) that the aggregate
               principal amount of Bonds outstanding at the date of such Written
               Order of the Company, and stating that the Fair Value of all of
               the Trust Estate (excluding the Trust Estate to be released but
               including any Property Additions to be acquired by the Company
               with the proceeds of, or otherwise in connection with, such
               release) stated on the Engineer's certificate filed pursuant to
               Clause (2) of Paragraph B equals or exceeds an amount equal to
               twenty-fourteenths (20/14) of such aggregate principal amount,
               and (ii) that, to the knowledge of the signer, no Event of
               Default has occurred and is continuing; and

                    (5)  an Opinion of Counsel complying with the provisions of
               Section 1.02 stating that the instruments which have been or are
               delivered to the Trustee conform to the requirements of this
               Indenture and constitute sufficient authority under this
               Indenture for the Trustee to execute and deliver the release
               requested.


               C.  Release up to a Limited Amount.  If the Company is unable, or
                   ------------------------------                               
          elects not, to obtain, in accordance with the preceding Paragraph B,
          the release from the Lien of this Indenture of any of the Trust
          Estate, unless an Event of Default shall have occurred and be
          continuing, upon receipt of a Written Order of the Company requesting
          the release of any of the Trust Estate pursuant to this Paragraph C,
          the Trustee shall execute and deliver to the Company the documents and
          instruments described in Clause (1) of Paragraph C releasing from the
          Lien of this Indenture any of the Trust Estate if the Fair Value
          thereof, as stated on the Engineer's certificate delivered pursuant to
          Clause (2) of Paragraph C is less than one percent (1%) of the
          aggregate principal amount of Bonds outstanding at the date of

                                       15
<PAGE>
 
          such Written Order of the Company, provided that the aggregate Fair
          Value of all Trust Estate released pursuant to this Paragraph C, as
          stated on all Engineer's certificates filed pursuant to this Paragraph
          C in any period of 12 consecutive calendar months which includes the
          date of such Engineer's certificate, shall not exceed three percent
          (3%) of the aggregate principal amount of Bonds outstanding at the
          date of such Written Order of the Company as stated in the Certificate
          of the Company delivered pursuant to Clause (3) of Paragraph C, upon
          receipt by the Trustee of:

                    (1)  appropriate documents and instruments releasing without
               recourse the interest of the Trustee in the Trust Estate to be
               released, and describing in reasonable detail the Trust Estate to
               be released;

                    (2)  an Engineer's certificate, dated the date of such
               Written Order of the Company, stating (i) that the signer of such
               Engineer's certificate has examined the Certificate of the
               Company delivered pursuant to Clause (2) of Paragraph C in
               connection with such release, (ii) the Fair Value, in the opinion
               of the signers of such Engineer's certificate, of such Trust
               Estate to be released as of a date not more than 90 days prior to
               the date of such Written Order of the Company, and (iii) that in
               the judgment of such signers, such release (A) will not
               materially adversely affect the Company's Electric Business and
               (B) will not impair the security under this Indenture in
               contravention of the provisions hereof;

                    (3)  a Certificate of the Company, dated the date of such
               Written Order of the Company, stating (i) the aggregate principal
               amount of Bonds outstanding at the date of such Written Order of
               the Company, (ii) that one percent (1%) of such aggregate
               principal amount exceeds the Fair Value of the Trust Estate for
               which such release is applied for, (iii) that three percent (3%)
               of such aggregate principal amount exceeds the aggregate Fair
               Value of all Trust Estate released from the Lien of this
               Indenture pursuant to this Paragraph C, as shown by all
               Engineer's certificates filed pursuant to Clause (2) of Paragraph
               C in such period of 12 consecutive calendar months, and (iv)
               that, to the knowledge of the signer, no Event of Default has
               occurred and is continuing; and

                                       16
<PAGE>
 
                    (4) an Opinion of Counsel complying with the provisions of
               Section 1.02 stating that the instruments which have been or are
               delivered to the Trustee conform to the requirements of this
               Indenture and constitute sufficient authority under this
               Indenture for the Trustee to execute and deliver the release
               requested.


               D.  Release by Deposit of Cash, Purchase Money Obligations or
                   ---------------------------------------------------------
          Property Additions.  If the Company is unable, or elects not, to
          ------------------                                              
          obtain, in accordance with Paragraphs B or C, the release from the
          Lien of this Indenture of any of the Trust Estate, unless an Event of
          Default shall have occurred and be continuing, upon receipt and
          deposit of a Written Order of the Company requesting the release of
          any of the Trust Estate pursuant to this Paragraph D and those items
          at Clause (2) in this Paragraph D, the Trustee shall execute and
          deliver to the Company the documents and instruments described in
          Clause (1) of Paragraph D releasing from the Lien of this Indenture
          the Trust Estate described in the Written Order of the Company.

                    (1)  appropriate documents and instruments releasing without
               recourse the interests of the Trustee in the Trust Estate to be
               released, and describing in reasonable detail the Trust Estate to
               be released;

                    (2)  Cash in an amount equal to the greater of the following
               items (i) and (ii):

                         (i)  the Fair Value of the property to be released, or

                         (ii) the consideration received or to be received by
                    the Company therefor (valuing purchase money obligations at
                    their principal amount and property received in exchange at
                    its Fair Value as stated in said certificate),

          provided, however, that in lieu of all or any part of such cash, the
          Company shall have the right to deposit with or deliver to the Trustee
          any of the following:

                    (a)  Purchase Money Obligations secured by a mortgage on the
               property to be released, or a portion thereof,

                                       17
<PAGE>
 
               not exceeding in principal amount seventy percent (70%) of the
               Fair Value (as certified as above set forth) of the property
               covered by such purchase money mortgage, which purchase money
               obligations and the mortgages securing the same, shall be duly
               assigned to the Trustee and shall be received by the Trustee at
               the principal amount thereof in lieu of cash; provided, however,
               that the Trustee shall not accept any such purchase money
               obligations in lieu of cash as provided in this Clause if thereby
               the aggregate principal amount of all purchase money obligations
               received by the Trustee pursuant to this Clause and at the time
               held by the Trustee would equal or exceed 10 percent of the
               principal amount of all Bonds then outstanding hereunder.

                    (b)  A Certificate of the trustee or other holder of a Prior
               Lien on all or any part of the property to be released, stating
               that a specific amount of cash and/or a specified principal
               amount of purchase money obligations of the character described
               in subparagraph (a) of this Clause and representing proceeds of
               the sale of such property, have been deposited with such trustee
               or other holder pursuant to the requirements of such Prior Lien,
               provided, however, that the aggregate of the cash and principal
               amount of purchase money obligations so certified at any one time
               shall in no event exceed the principal amount of the Prior Lien
               Obligations outstanding thereunder, less any amounts then held by
               the trustee or other holder of such Prior Lien other than for the
               payment or redemption of Prior Lien Obligations not deemed
               outstanding under the provisions of Section 4.01; and such
               certificate shall be received by the Trustee in lieu of cash
               equal to the cash and the principal amount of the purchase money
               obligations so certified to have been deposited with such trustee
               or other holder of such Prior Lien.

                    (c)  The Certificates, Opinions and Other Instruments which
               the Company would be required to furnish to the Trustee, upon an
               application for the authentication and delivery of Bonds on the
               basis of Property Additions under Article Four, but with the
               following variations and omissions of the instruments specified
               in Section 4:02:

                                       18
<PAGE>
 
                         (i) There shall be an additional statement in Clause
                    (2) of the Property Additions Certificate, to the effect
                    that no part of the Property Additions therein described has
                    in any other previous or then pending application been made
                    the basis for the release of any Unbonded Property from the
                    lien of this Indenture or for the withdrawal of any Unbonded
                    Cash from the Trustee or from the trustee or other holder of
                    a Prior Lien, or to repair, replace, or restore insured
                    Unbonded Property which shall have been damaged or destroyed
                    but the proceeds of the insurance on which shall not have
                    been required to be paid to the Trustee pursuant to the
                    provisions of Section 9.10;

                         (ii)  It shall not be necessary for the Company to
                    deliver to the Trustee the Resolution required by Paragraph
                    A, the Retirements Certificate required by Paragraph E, the
                    Net Earnings Certificate required by Paragraph F, or any of
                    the certificates or parts of the Opinion of Counsel referred
                    to in Clauses (6) and (7) of Paragraph I of Section 4.02;

                         (iii)  The Summary Certificate required by Paragraph G
                    of Section 4.02 shall show only Gross Bondable Additions and
                    may include any Additions Credit; and

                         (iv)  If no part of the property to be released is
                    Bonded Property and such property or any part thereof is
                    subject to a Prior Lien, the Property Additions then so
                    certified may be subject to the same Prior Lien, and the
                    Property Additions Certificate required by Paragraph B of
                    Section 4.02 and the Opinion of Counsel required by
                    Paragraph I of Section 4.02 may be modified accordingly.

                    Such Certificates, Opinions and Other Instruments shall be
               received by the Trustee in lieu of cash up to the amount of the
               Gross Bondable Additions so certified to the Trustee.

                    (3)  An Opinion or Opinions of Counsel, complying with the
               provisions of Section 1.02,

                                       19
<PAGE>
 
                         (a) stating that the instruments which have been or are
                    therewith delivered to the Trustee conform to the
                    requirements of this Indenture and constitute sufficient
                    authority under this Indenture for the Trustee to execute
                    and deliver the release requested, and that, upon the basis
                    of the cash, purchase money obligations, certificates,
                    opinions and other instruments delivered to the Trustee
                    pursuant to Paragraph D of this Section, the property so
                    sold or disposed of or contracted to be sold or disposed of
                    may lawfully be released from the lien of this Indenture
                    pursuant to the provisions of this Section;

                         (b)  stating that the purchase money obligations, if
                    any, delivered to the Trustee or to the trustee or other
                    holder of a Prior Lien pursuant to subparagraph (a) of
                    Clause (2) of Paragraph D of this Section are valid
                    obligations and are duly secured by a valid purchase money
                    mortgage constituting a direct lien upon all the property to
                    be released, or upon the portion thereof described, free and
                    clear of all prior liens, charges or encumbrances, except
                    any Prior Liens or other charges or encumbrances prior to
                    the lien of this Indenture which may have existed on the
                    property to be released immediately prior to such release
                    and that the assignment of any mortgage securing such
                    purchase money obligations is valid and in recordable form;
                    and

                         (c)  in case, pursuant to subparagraph (a) of Clause
                    (2) of Paragraph D of this Section, any cash or purchase
                    money obligations shall be certified to have been deposited
                    with the trustee or other holder of a Prior Lien, stating
                    that the property to be released, or a specified portion
                    thereof, is or immediately before such sale or disposition
                    was subject to such Prior Lien and that such deposit is
                    required by such Prior Lien.

               E.  References to Other Sections of Article Seven.  In Section
                   ---------------------------------------------             
          7.04 of the Indenture, the reference to Paragraph B of Section 7.02
          and Clause (8) is deleted.  In Section 7.05 of the Indenture, the
          reference to the Resolution of the Board required by Section 7.02
          shall mean Written Order of the Company.

                                       20
<PAGE>
 
     1.14.  Deletion of Release up to $5,000.  Section 7.08 of Article Seven of
            --------------------------------                                   
the Indenture is hereby deleted from the Indenture.  All references to Section
7.08 in Article Seven of the Indenture are deleted from the Indenture.


     1.15.  Amendment of Redemption of Bonds Because of Eminent Domain.  Section
            ----------------------------------------------------------          
8.08(b) of Article Eight of the Indenture is amended so as to read as follows:

          (b)  In case of the sale and release of, or the taking by eminent
     domain or of the purchase by a public authority (pursuant to any right
     which it may then have to make such purchase) of the entire Trust Estate,
     then all Trust Moneys representing the proceeds thereof received by the
     Trustee shall be applied by the Trustee in accordance with the provisions
     of Section 8.05 to the redemption of Bonds outstanding hereunder (prorated
     between or among the several series, according to the principal amount of
     Bonds outstanding of each series, if Bonds of more than one series be
     outstanding) at such then applicable redemption prices, in such manner and
     upon such notice (which shall be given by the Trustee for and on behalf of
     the Company, and in the name of the Company) as may be specified in respect
     of said Bonds of each series in this Indenture or in any applicable
     indenture supplemental hereto.


     1.16.  Deletion of 15% Maintenance Requirement.  Section 9.06 of Article
            ---------------------------------------                          
Nine of the Indenture is hereby deleted from the Indenture.


     1.17.  Increasing Insured Loss Retention.  The third sentence of Section
            ---------------------------------                                
9.10 of Article Nine of the Indenture as previously amended by Section 1.04 of
the Fifteenth Supplemental Indenture is further amended so as to read as
follows:

          All policies or other contracts for insurance upon the Trust Estate
     shall provide that any loss in excess of Five Million Dollars shall be
     payable to the Trustee as its interest may appear, or to the trustee or
     other holder of any Prior Lien if required by the terms thereof; and, if so
     requested in writing by the Trustee, the Company will, subject to the
     provisions of any Prior Lien, cause policies for such insurance to be
     delivered to the Trustee.

     The last paragraph of Section 9.10 of Article Nine of the Indenture as
previously amended by Section 1.05 of the Fifteenth Supplemental Indenture is
further amended by substituting the number $5,000,000 for the number $1,000,000
therein.

                                       21
<PAGE>
 
     1.18.  Deletion of Limitations on Prior Liens.  Section 9.15 of Article
            --------------------------------------                          
Nine of the Indenture is hereby deleted from the Indenture.


     1.19.  Restrictions on Dividends.  Section 9.16 of Article Nine of the
            -------------------------                                      
Indenture as amended by Section 3.06 of the Third Supplemental Indenture,
Section 1.06 of the Fifteenth Supplemental Indenture and Section 1.02 of the
Twenty Second Supplemental Indenture is further amended so as to read as
follows:

          Other than dividends payable solely in shares of its common stock, the
     Company may declare and pay dividends in cash or property on any shares of
     its common stock only out of the unreserved and unrestricted retained
     earnings of the Company and shall not make any such declaration or payment
     when the Company is insolvent, or when the payment thereof would render the
     Company insolvent.


     1.20.  Deletion of Restrictions on Investments in Affiliates and
            ---------------------------------------------------------
Prohibiting Subsidiaries from Engaging in Electric Business.  Section 9.20 of
- -----------------------------------------------------------                  
Article Nine of the Indenture is hereby deleted from the Indenture.


     1.21.  Deletion of Restrictions on Investments in Subsidiaries.  Section
            -------------------------------------------------------          
9.21 of Article Nine of the Indenture as amended by Section 1.07 of the
Fifteenth Supplemental Indenture, Section 1.06 of the Eighteenth Supplemental
Indenture, Section 1.03 of the Twenty Second Supplemental Indenture and Section
1.02 of the Twenty Fourth Supplemental Indenture is hereby deleted from the
Indenture.


     1.22.  Annual Compliance Certificate to Conform to the Amendments.  Section
            ----------------------------------------------------------          
9.22 of Article Nine of the Indenture is amended so as to read as follows:

          SECTION 9.22.  On or before May 1 in each calendar year, or on or
     before such other date in each calendar year as the Company and the Trustee
     may agree upon, the Company will deliver to the Trustee a Certificate of
     the Company, complying with the provisions of Section 1.02, with respect to
     the compliance by the Company with the covenants contained in Sections
     9.04, 9.05, 9.07, 9.08, 9.11, 9.12, and 9.13, and the Company covenants and
     agrees to notify the Trustee immediately upon the occurrence of any event
     which constitutes an Event of Default (as defined in Section 12.01 hereof)
     or which may constitute an Event of Default as the result of the giving of
     a notice and/or expiration of a period of grace.

                                       22
<PAGE>
 
     1.23.  Noncertificated System for Registration and Restatement of the
            --------------------------------------------------------------
Indenture.  Section 17.01 of the Indenture is amended so as to add thereto the
- ---------                                                                     
following additional Paragraphs J and K:

               J.  To provide for the procedures required to permit the Company
          to utilize, at its option, a noncertificated system of registration
          for all, or any series of the Bonds.

               K.  To enter into a restatement of the Indenture without material
          modifications and including all amendments contained in supplements
          that remain in effect, with authority to reorganize material, renumber
          and letter, include reference headings and remove language no longer
          applicable and clarify any ambiguities in the Indenture as amended.


                                  ARTICLE TWO

                            Miscellaneous Provisions

     2.01.  Headings at the beginning of each Section are included for
convenience of reference, and such headings are not intended to be used to
interpret this Supplemental Indenture.

     2.02.  The Company, by the execution hereof, acknowledges that a true copy
of this Supplemental Indenture has been delivered to and received by it.

     2.03.  Except as heretofore amended and as amended by this Supplemental
Indenture, all the provisions, terms and conditions of the Original Indenture
shall continue in full force and effect.

     2.04.  The recitals contained in this Supplemental Indenture shall be taken
as the statements of the Company, and the Trustee assumes no responsibility for
their correctness.

     2.05.  This Supplemental Indenture may be executed in several counterparts,
all or any of which may be treated for all purposes as one original and shall
constitute and be one and the same instrument.

     IN WITNESS WHEREOF, BLACK HILLS CORPORATION, party hereto of the first
part, has caused this Supplemental Indenture to be executed on its behalf by its

                                       23
<PAGE>
 
President or one of its Vice Presidents and its corporate seal to be hereto
affixed and to be attested by its Secretary or an Assistant Secretary, and
CHEMICAL BANK, party hereto of the second part, in evidence of its acceptance of
the trust hereby created, has caused this Supplemental Indenture to be executed
on its behalf by one of its Vice Presidents or Assistant Vice Presidents and its
corporate seal to be hereto affixed and to be attested by its Secretary or an
Assistant Secretary or a Trust Officer duly authorized, all as of the day and
year first above written.

                              BLACK HILLS CORPORATION
(CORPORATE SEAL)

                              By________________________________________________

                                 Its____________________________________________
 
ATTEST:

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

Signed, sealed and delivered by
BLACK HILLS CORPORATION
in the presence of


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


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

                                       24
<PAGE>
 
                              CHEMICAL BANK


                              By________________________________________________
 
                                 Its____________________________________________
 
ATTEST:

- --------------------------------------
 
Its___________________________

Signed, sealed and delivered by
CHEMICAL BANK
in the presence of:


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


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


STATE OF SOUTH DAKOTA

COUNTY OF PENNINGTON

     On the _____ day of ___________, 1994, before me,________________________,
the undersigned officer, personally appeared ______________________, to me
personally known, who acknowledged h__self to be, and being by me duly sworn,
did say that __he is _______________________ of BLACK HILLS CORPORATION, a
corporation, and that the seal affixed to the foregoing instrument is the
corporate seal of said corporation and that said instrument was executed by, and
signed in the name of, the corporation, by h__, as such
__________________________ and sealed in behalf of the corporation by authority
of its Board of Directors for the purposes therein contained, and the said

                                       25
<PAGE>
 
________________________ acknowledged the same as the free act and deed of said
corporation.

     IN WITNESS WHEREOF, I hereunto set my hand and official seal.



                                  ----------------------------------------------
                                  NOTARY PUBLIC
(SEAL)


STATE OF NEW YORK

COUNTY OF NEW YORK

     On this _______ day of ________________, 1994, before me,
____________________, the undersigned officer, personally appeared
____________________, to me personally known, who acknowledged himself to be,
and being by me duly sworn, did say that he is ______________________________ of
CHEMICAL BANK, a corporation, and that the seal affixed to the foregoing
instrument is the corporate seal of said corporation and that said instrument
was executed by, and signed in the name of, the corporation, by him, as such
_____________________________ and sealed in behalf of the corporation by
authority of its Board of Directors for the purposes therein contained, and the
said ______________________ acknowledged the same as the free act and deed of
said corporation.

     IN WITNESS WHEREOF, I hereunto set my hand and official seal.



                                  ----------------------------------------------
                                  NOTARY PUBLIC
(SEAL)
 

                                       26

<PAGE>
  
EXHIBIT 5.01


MORRILL BROWN & THOMAS
625 Ninth Street - 8th Floor
P. O. Box 8108
Rapid City, South Dakota  57709-8108
605-348-7516



                                June 28, 1994



Board of Directors of
     Black Hills Corporation
625 Ninth Street
P. O. Box 1400
Rapid City, SD  57709

     Re:  Opinion of Counsel
          Black Hills Corporation
          Form S-3
          $100,000,000 of First Mortgage Bonds


     Black Hills Corporation ("Company") is filing with the Securities and
Exchange Commission and Registration Statement on Form S-3 ("Registration
Statement") for the issuance and sale of up to $100,000,000 of First Mortgage
Bonds to be issued in one or more series under, and all equally and ratably
secured by an Indenture of Mortgage and Deed of Trust dated as of September 1,
1941, and supplemented by twenty-seven Supplemental Indentures ("Indenture").
We are general counsel for the Company and represent the Company in connection
with the issuance of the New Bonds.

     We are of the opinion that:

     1.   The Company is a corporation validly incorporated and existing under
the laws of the State of South Dakota and is fully qualified to carry on the
business in which it is now engaged.

     2.   The Indenture has been duly and validly authorized, executed and
delivered by the Company and, as amended by the Trust Indenture Reform Act of
1990, constitutes a valid binding agreement of the Company, enforceable in
accordance with its terms.
<PAGE>
 
     2.   When the Board of Directors has authorized the issuance and sale of a
series of the New Bonds in accordance with the Indenture and adopted a
Supplemental Indenture determining the terms and provisions thereof, and when
such New Bonds have been executed, authenticated and delivered in accordance
with the terms of the Indenture and such Supplemental Indenture, and as
described in the Registration Statement and in the Underwriting Agreement filed
as an exhibit thereto, each such New Bond will be subject to the lien of the
Indenture and will constitute a valid and legally binding obligation of the
Company enforceable in accordance with its terms, except as enforcement thereof
may be limited by bankruptcy, insolvency or other laws relating to or affecting
enforcement of creditors' rights or by general equity principles.

     We hereby consent to the filing of a copy of this opinion as an exhibit to
the Registration Statement.  We also consent to the use of this firm's name in
the Registration Statement and the Prospectus constituting a part thereof.

                                Respectfully submitted,



                                /s/ Morrill Brown & Thomas

DEM:br


<PAGE>
 
RATIO OF EARNINGS TO FIXED CHARGES
                                                                  EXHIBIT 12.01
(amounts in thousands)

<TABLE>
<CAPTION>
                                                                      Twelve    Three
                                    Year Ended December 31            Months    Months
                         -----------------------------------------     Ended     Ended
                          1989     1990     1991     1992     1993    3/31/94   3/31/94
                          ----     ----     ----     ----     ----    -------   -------
<S>                      <C>      <C>      <C>      <C>      <C>      <C>       <C>
Pre-Tax Income From
 Continuing Operations   $30,288  $31,675  $31,230  $32,144  $31,911   $31,562  $ 8,171
                         -------  -------  -------  -------  -------   -------  -------
Fixed Charges:
 Interest Expense          4,304    4,951    7,922    8,860    8,699     8,681    2,118
 Amortization of
  Debt Expense                71       69       79      105      118       117       29
 Imputed Interest From
  Capitalized Lease        2,960    2,851      761        -        -         -        -
                         -------  -------  -------  -------  -------   -------  -------
                           7,335    7,871    8,762    8,965    8,817     8,798    2,147
                         -------  -------  -------  -------  -------   -------  -------
Total Earnings Plus
 Fixed Charges           $37,623  $39,546  $39,992  $41,109  $40,728   $40,360  $10,318
                         =======  =======  =======  =======  =======   =======  =======
Total Fixed Charges      $ 7,335  $ 7,871  $ 8,762  $ 8,965  $ 8,817   $ 8,798  $ 2,147
                         =======  =======  =======  =======  =======   =======  =======
Ratio of Earnings To
Fixed Charges                5.1      5.0      4.6      4.6      4.6       4.6      4.8
</TABLE>

<PAGE>
 
                                                                   Exhibit 23.01



                   CONSENT OF INDEPENDENT PUBLIC ACCOUNTANTS


As independent public accountants, we hereby consent to the incorporation by
reference in this registration statement of our reports dated January 28, 1994
included in Black Hills Corporation Annual Report on Form 10-K for the year
ended December 31, 1993 and to all references to our Firm included in this
registration statement.



                                                       ARTHUR ANDERSEN & CO.



Minneapolis, Minnesota,
 June 28, 1994

<PAGE>
 
      ___________________________________________________________________

                       SECURITIES AND EXCHANGE COMMISSION
                            Washington, D. C.  20549
                           _________________________

                                   FORM  T-1

                            STATEMENT OF ELIGIBILITY
                    UNDER THE TRUST INDENTURE ACT OF 1939 OF
                   A CORPORATION DESIGNATED TO ACT AS TRUSTEE
                  ___________________________________________
              CHECK IF AN APPLICATION TO DETERMINE ELIGIBILITY OF
                A TRUSTEE PURSUANT TO SECTION 305(b)(2) ________
                    ________________________________________

                                 CHEMICAL BANK
              (Exact name of trustee as specified in its charter)

New York                                                     13-4994650
(State of incorporation                                (I.R.S. employer
if not a national bank)                             identification No.)

270 Park Avenue
New York, New York                                                10017
(Address of principal executive offices)                     (Zip Code)

                               William H. McDavid
                                General Counsel
                                270 Park Avenue
                            New York, New York 10017
                              Tel:  (212) 270-2611
           (Name, address and telephone number of agent for service)
                 _____________________________________________
                            Black Hills Corporation
              (Exact name of obligor as specified in its charter)

South Dakota                                                 46-0111677
(State or other jurisdiction of                        (I.R.S. employer
incorporation or organization)                      identification No.)

625 Ninth Street
Rapid City, South Dakota                                          57709
(Address of principal executive offices)                     (Zip Code)

                  ___________________________________________
                              First Mortgage Bonds
                      (Title of the indenture securities)
             _____________________________________________________
<PAGE>
 
                                    GENERAL

Item 1. General Information.

        Furnish the following information as to the trustee:

        (a)  Name and address of each examining or supervising authority to
        which it is subject. New York State Banking Department, State House,
        Albany, New York  12110.

        Board of Governors of the Federal Reserve System, Washington, D.C.,
        20551 and Federal Reserve Bank of New York, District No. 2, 33 Liberty
        Street, New York, N.Y.

        Federal Deposit Insurance Corporation, Washington, D.C., 20429.

        (b)  Whether it is authorized to exercise corporate trust powers.

             Yes.

Item 2. Affiliations with the Obligor.

        If the obligor is an affiliate of the trustee, describe each such
        affiliation.

        None.

                                       2
<PAGE>
 
Item 16. List of Exhibits
 
         List below all exhibits filed as a part of this Statement of
         Eligibility.

         1.  A copy of the Articles of Association of the Trustee as now in
effect, including the  Organization Certificate and the Certificates of
Amendment dated February 17, 1969, August 31, 1977, December 31, 1980,
September 9, 1982, February 28, 1985 and December 2, 1991 (see Exhibit 1 to
Form T-1 filed in connection with Registration Statement  No. 33-50010, which
is incorporated by reference).

         2.  A copy of the Certificate of Authority of the Trustee to Commence
Business (see Exhibit 2 to Form T-1 filed in connection with Registration
Statement No. 33-50010, which is incorporated by reference).

         3.  None, authorization to exercise corporate trust powers being
contained in the documents identified above as Exhibits 1 and 2.

         4.  A copy of the existing By-Laws of the Trustee (see Exhibit 4 to
Form T-1 filed in connection with Registration Statement No. 33-46892, which is
incorporated by reference).

         5.  Not applicable.

         6.  The consent of the Trustee required by Section 321(b) of the Act
(see Exhibit 6 to Form T-1 filed in connection with Registration Statement No.
33- 50010, which is incorporated by reference).

         7.  A copy of the latest report of condition of the Trustee, published
pursuant to law or the requirements of its supervising or examining authority.

         8.  Not applicable.

         9.  Not applicable.

                                   SIGNATURE

      Pursuant to the requirements of the Trust Indenture Act of 1939 the
Trustee, Chemical Bank, a corporation organized and existing under the laws of
the State of New York, has duly caused this statement of eligibility to be
signed on its behalf by the undersigned, thereunto duly authorized, all in the
City of New York and State of New York, on the 15th day of June, 1994.
 
                                          CHEMICAL BANK

 
                                          By  /s/ T. C. Knight
                                            ----------------------------------
                                                  T. C. Knight
                                                  Assistant Vice President
 

                                       3
<PAGE>
 
                             Exhibit 7 to Form T-1


                                Bank Call Notice

                             RESERVE DISTRICT NO. 2
                      CONSOLIDATED REPORT OF CONDITION OF

                                 Chemical Bank
                  of 270 Park Avenue, New York, New York 10017
                     and Foreign and Domestic Subsidiaries,
                    a member of the Federal Reserve System,

             at the close of business March 31, 1994, published in
        accordance with a call made by the Federal Reserve Bank of this
        District pursuant to the provisions of the Federal Reserve Act.

<TABLE>
<CAPTION>
                                                       Dollar Amounts
            ASSETS                                       in Millions
<S>                                             <C>        <C>
Cash and balances due from depository institutions:
  Noninterest-bearing balances and
  currency and coin .................................      $  5,741
  Interest-bearing balances .........................         3,768
Securities ..........................................
Held to maturity securities..........................         7,503
Available for sale securities........................        15,662
Federal Funds sold and securities purchased under
  agreements to resell in domestic offices of the
  bank and of its Edge and Agreement subsidiaries,
  and in IBF's:
  Federal funds sold.................................         2,514
  Securities purchased under agreements to resell....           995
Loans and lease financing receivables:
  Loans and leases, net of unearned income....  $61,140
  Less: Allowance for loan and lease losses...    2,315
  Less: Allocated transfer risk reserve.......      115
                                                -------
  Loans and leases, net of unearned income,
  allowance, and reserve.............................        58,710
Assets held in trading accounts......................        26,249
Premises and fixed assets (including capitalized
  leases)............................................         1,310
Other real estate owned..............................           642
Investments in unconsolidated subsidiaries and
  associated companies...............................           120
Customer's liability to this bank on acceptance
  outstanding........................................         1,093
Intangible assets....................................           549
Other assets.........................................         7,807
                                                            -------
TOTAL ASSETS.........................................      $132,663
                                                           ========
</TABLE>

                                       4
<PAGE>
                                  LIABILITIES

<TABLE>
<S>                                           <C>           <C>
Deposits
 In domestic offices...................................... $ 49,180
 Noninterest-bearing .........................$16,896
 Interest-bearing ............................ 32,284
                                              -------
 In foreign offices, Edge and Agreement subsidiaries,
 and IBF's................................................   25,612
 Noninterest-bearing .........................$   128
 Interest-bearing ............................ 25,484
                                              -------
Federal funds purchased and securities sold under agree-
ments to repurchase in domestic offices of the bank and
 of its Edge and Agreement subsidiaries, and in IBF's
 Federal funds purchased..................................   10,710
 Securities sold under agreements to repurchase...........    1,789
Demand notes issued to the U.S. Treasury..................    1,493
Trading liabilities.......................................   14,745
Other Borrowed money:
 with original maturity of one year or less...............    6,331
 with original maturity of more than one year.............    1,031
Mortgage indebtedness and obligations under capitalized
 leases...................................................       21
Bank's liability on acceptances executed and outstanding      1,096
Subordinated notes and debentures.........................    3,500
Other liabilities.........................................    9,562
 
TOTAL LIABILITIES.........................................  125,070
                                                           --------

                                 EQUITY CAPITAL
Common stock..............................................      620
Surplus...................................................    4,501
Undivided profits and capital reserves....................    2,684
Less: Net unrealized loss on marketable equity
       securities.........................................     (210)
Cumulative foreign currency translation adjustments.......       (2)
 
TOTAL EQUITY CAPITAL......................................    7,593
                                                           --------
 
TOTAL LIABILITIES, LIMITED-LIFE PREFERRED
    STOCK AND EQUITY CAPITAL.............................. $132,663
                                                           ========
</TABLE>

I, Joseph L. Sclafani, S.V.P. & Controller of the
above-named bank, do hereby declare that this Report of
Condition is true and correct to the best of my knowledge
and belief.

                    JOSEPH L. SCLAFANI

We, the undersigned directors, attest to the correctness
of this statement of resources and liabilities.  We
declare that it has been examined by us, and to the best
of our knowledge and belief has been prepared in confor-
mance with the instructions and is true and correct.


                    WALTER V. SHIPLEY       )
                    EDWARD D. MILLER        )DIRECTORS
                    WILLIAM B. HARRISON     )

                                       5


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