EXHIBIT 3.6

LIMITED LIABILITY COMPANY AGREEMENT

ARCH OF WYOMING, LLC

This Limited Liability Company Agreement (this "Agreement") of
Arch of Wyoming, LLC is entered into by Arch Coal, Inc., a Delaware corporation
(the "Member").

The Member, by execution of this Agreement, hereby forms a
limited liability company pursuant to and in accordance with the Delaware
Limited Liability Company Act (6 Del. C Section 18-101, et seq.), as amended
from time to time (the "Act"), and hereby agrees as follows:

1. Name. The name of the limited liability company
formed hereby is Arch of Wyoming, LLC (the "Company").

2. Certificates. Miriam Rogers Singer, as an authorized
person within the meaning of the Act, shall execute, deliver and file the
Certificate of Formation with the Secretary of State of the State of Delaware.
Upon the filing of the Certificate of Formation with the Secretary of State of
the State of Delaware, her powers as an authorized person shall cease and the
Member shall thereafter be designated as an authorized person within the meaning
of the Act. The member or an Officer shall execute, deliver and file any other
certificates (and any amendments and/or restatements thereof) necessary for the
Company to qualify to do business in a jurisdiction in which the Company may
wish to conduct business.

3. Purposes. The Company shall have all powers now or
hereafter conferred by the laws of the State of Delaware on limited liability
companies formed under the Act and, subject to the terms of this Agreement, may
do any and all lawful acts or things that are necessary, appropriate, incidental
or convenient for the furtherance and accomplishment of the purposes of the
Company. Without limiting the generality of the foregoing, the Company may enter
into, deliver and perform all contracts, agreements and other undertakings and
engage in all activities and transactions as may be necessary or appropriate to
carry out its purposes.

4. Principal Business Office. The principal business
office of the Company shall be located at such
location as may hereafter be determined by the Member.

5. Registered Office. The address of the registered
office of the Company in the State of Delaware is c/o The Corporation Trust
Company, Corporation Trust Center, 1209 Orange Street, Wilmington, New Castle
county, DE 19801.

6. Registered Agent. The name and address of the
registered agent of the Company for service of process on the Company in the
State of Delaware is The

Corporation Trust Company, Corporation Trust Center, 1209 Orange Street,
Wilmington, New Castle County, DE 19801.

7. Member. The name and the mailing address of the
Member is set forth on Schedule A attached hereto.

8. Limited Liability. Except as otherwise provided by
the Act, the debts, obligations and liabilities of the Company, whether arising
in contract, tort or otherwise, shall be solely the debts, obligations and
liabilities of the Company, and the Member shall not be obligated personally for
any such debt, obligation or liability of the Company solely by reason of being
a member of the Company.

9. Capital Contributions. The Member is deemed admitted
as the Member of the Company upon its execution and delivery of this Agreement.
The Member will contribute the amount of United States Dollars to the Company as
listed on Schedule A attached hereto.

10. Additional Contributions. The Member is not required
to make any additional capital contribution to the Company. However, a Member
may make additional capital contributions to the Company at such times and in
such amounts as determined by the member.

11. Allocation of Profits and Losses. The Company's
profits and losses shall be allocated to the Member.

12. Distributions. Distributions shall be made to the
Member at the times and in the aggregate amounts determined by the Member.
Notwithstanding any provision to the contrary contained in this Agreement, the
Company shall not make a distribution to any Member on account of its interests
in the Company if such distribution would violate Section 18-607 of the Act or
other applicable law.

13. Management. In accordance with Section 18-402 of the
Act, management of the Company shall be vested in the Member. The Member shall
have the power to do any and all acts necessary, convenient or incidental to or
for the furtherance of the purposes described herein, including all powers,
statutory or otherwise, possessed by members of a limited liability company
under the laws of the State of Delaware. The Member has the authority to bind
the Company.

14. Officers. The Member may, from time to time as it
deems advisable, appoint officers of the Company (the "Officers") and assign in
writing titles (including, without limitation, President, Vice President,
Secretary, and Treasurer) to any such person. Unless the Member decides
otherwise, if the title is one commonly used for officers of a business
corporation formed under the Delaware General Corporation Law, the assignment of
such title shall constitute the delegation to such person of the authorities and
duties that are normally associated with that office. Any delegation pursuant to
this Section 14 may be revoked at any time by the Member. The initial Officers
are listed on Schedule B attached hereto. The Member may revise Schedule B in
its sole discretion at any time.

15. Other business. The Member may engage in or possess
an interest in other business ventures (unconnected with the Company) of every
kind and description, independently or with others. The Company shall not have
any rights in or to such independent ventures or the income or profits therefrom
by virtue of this Agreement.

16. Exculpation and Indemnification. No Member, no
Affiliate of a member, nor any Officer, employee or agent of the Company
("Indemnified Party") shall be liable to the Company, or any other person or
entity who has an interest in the Company, for any loss, damage or claim
incurred by reason of any act or omission performed or omitted by such Member or
Officer in good faith on behalf of the Company and in a manner reasonably
believed to be within the scope of the authority conferred on such Member or
Officer by this Agreement, unless such act or omission constituted bad faith,
gross negligence, fraud or wilful misconduct. To the full extent permitted by
applicable law, an Indemnified Party shall be entitled to indemnification from
the Company for any loss, damage or claim incurred by an Indemnified Party by
reason of any act or omission performed or omitted by an Indemnified Party in
good faith on behalf o the Company and in a manner reasonably believed to be
within the scope of the authority conferred an Indemnified Party by this
Agreement, unless such act or omission constituted bad faith, gross negligence,
fraud or wilful misconduct; provided, however, that any indemnity under this
Section 16 shall be provided out of and to the extent of Company assets only,
and no Member shall have personal liability on account thereof.

17. Assignments. A Member may assign in whole or in part
its limited liability company interest with the written consent of the Member.
If a Member transfers all of its interest in the Company pursuant to this
Section, the transferee shall be admitted to the Company upon its execution of
an instrument signifying its agreement to be bound by the terms and conditions
of this Agreement. Such admission shall be deemed effective immediately prior to
the transfer, and, immediately following such admission, the transferor Member
shall cease to be a member of the Company.

18. Resignation. A Member may resign from the Company
with the written consent of the Member. If a Member is permitted to resign
pursuant to this Section, an additional member shall be admitted to the Company,
subject to Section19, upon its execution of an instrument signifying its
agreement to be bound by the terms and conditions of this Agreement. Such
admission shall be deemed effective immediately prior to the resignation, and,
immediately following such admission, the resigning Member shall cease to be a
member of the Company.

19. Admission of Additional Members. One or more
additional members of the Company may be admitted to the Company with the
written consent of the Member.

20. Dissolution.

a. The Company shall dissolve, and its affairs
shall be wound up upon the first to occur of the following: (i) the written
consent of the Member, (ii) the retirement, resignation or dissolution of the
Member or the occurrence of any other event which terminates the continued
membership of the Member in the Company unless the business of the Company is
continued in a manner permitted by the Act, or (iii) the entry of a decree of
judicial dissolution under Section 18-802 of the Act.

b. The bankruptcy of the Member will not cause
the Member to cease to be a member of the Company and upon the occurrence of
such an event, the business of the Company shall continue without dissolution.

c. In the event of dissolution, the Company
shall conduct only such activities as are necessary to wind up its affairs
(including the sale of the assets of the Company in an orderly manner), and the
assets of the Company shall be applied in the manner, and in the order of
priority, set forth in Section 18-804 of the Act.

21. Separability of Provisions. Each provision of this
Agreement shall be considered separable and if for any reasons any provision or
provisions herein are determined to be invalid, unenforceable or illegal under
any existing or future law, such invalidity, unenforceability or illegality
shall not impair the operation of or affect those portions of this Agreement
which are valid, enforceable and legal.

22. Counterparts. This Agreement may be executed in any
number of counterparts, each of which shall be deemed an original of this
Agreement.

23. Entire Agreement. This Agreement constitutes the
entire agreement of the Member with respect to the subject matter hereof.

24. Governing Law. This Agreement shall be governed by,
and construed under, the laws of the State of Delaware (without regard to
conflict of laws principles), all rights and remedies being governed by said
laws.

25. Amendments. This Agreement may not be modified,
altered, supplemented or amended except pursuant to a written agreement executed
and delivered by the Member.

IN WITNESS WHEREOF, the undersigned, intending to be legally
bound hereby, has duly executed this Agreement as of the ___ day of April, 1998.

Arch Coal, Inc.

By: /s/ JEFFRY N. QUINN
----------------------------
Jeffry N. Quinn
Senior Vice President

SCHEDULE A

ARCH OF WYOMING, LLC

LIMITED LIABILITY COMPANY AGREEMENT

MEMBER
- ------
Agreed Value of Percentage
Name Mailing Address Capital Contribution Interest
- ---- --------------- -------------------- ----------

Arch Coal, Inc. CityPlace One $1,000.00 100%
Suite 300
St. Louis, MO 63141

SCHEDULE B

ARCH OF WYOMING, LLC

LIMITED LIABILITY COMPANY AGREEMENT

NAME TITLE
- ---- -----

Paul Lang President and General Manager

Jeffry N. Quinn Vice President

David B. Peugh Vice President

Patrick Kriegshauser Vice President

Steven E. McCurdy Vice President

Mark Luzecky Vice President & Treasurer

Miriam Rogers Singer Secretary

William H. Rose Assistant Secretary