UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

o

Rule 13d-1(b)

x

Rule 13d-1(c)

o

Rule 13d-1(d)

1.

Names of Reporting Persons
Sirios Capital Partners, L.P.

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

x

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization:
Delaware

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
- 0 -

6.

Shared Voting Power
126,973

7.

Sole Dispositive Power
- 0 -

8.

Shared Dispositive Power
126,973

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
126,973

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

Not Applicable

11.

Percent of Class Represented by Amount in Row (9)
0.39%

12.

Type of Reporting Person (See Instructions)
PN

1.

Names of Reporting Persons
Sirios Capital Partners II, L.P.

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

x

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization:
Delaware

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
- 0 -

6.

Shared Voting Power
482,488

7.

Sole Dispositive Power
- 0 -

8.

Shared Dispositive Power
482,488

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
482,488 shares

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

Not applicable

11.

Percent of Class Represented by Amount in Row (9)
1.48%

12.

Type of Reporting Person (See Instructions)
PN

1.

Names of Reporting Persons
Sirios Overseas Fund, Ltd.

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

x

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization:
Cayman Islands

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
- 0 -

6.

Shared Voting Power
810,162

7.

Sole Dispositive Power
- 0 -

8.

Shared Dispositive Power
810,162

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
810,162 shares

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

Not applicable

11.

Percent of Class Represented by Amount in Row (9)
2.48%

12.

Type of Reporting Person (See Instructions)
OO

1.

Names of Reporting Persons
Sirios Focus Partners, L.P.

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

x

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization:
Cayman Islands

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
- 0 -

6.

Shared Voting Power
140,838

7.

Sole Dispositive Power
- 0 -

8.

Shared Dispositive Power
140,838

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
140,838 shares

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

Not applicable

11.

Percent of Class Represented by Amount in Row (9)
0.43%

12.

Type of Reporting Person (See Instructions)
PN

1.

Names of Reporting Persons
Sirios Capital Management, L.P.

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

x

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization:
Delaware

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
- 0 -

6.

Shared Voting Power
1,560,461

7.

Sole Dispositive Power
- 0 -

8.

Shared Dispositive Power
1,560,461

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
1,560,461 shares

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

Not applicable

11.

Percent of Class Represented by Amount in Row (9)
4.78%

12.

Type of Reporting Person (See Instructions)
IA, PN

1.

Names of Reporting Persons
Sirios Associates, L.L.C.

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

x

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization:
Delaware

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
- 0 -

6.

Shared Voting Power
1,560,461

7.

Sole Dispositive Power
- 0 -

8.

Shared Dispositive Power
1,560,461

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
1,560,461 shares

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

Not applicable

11.

Percent of Class Represented by Amount in Row (9)
4.78%

12.

Type of Reporting Person (See Instructions)
OO

1.

Names of Reporting Persons
John F. Brennan, Jr.

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

x

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization:
U.S.A.

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
- 0 -

6.

Shared Voting Power
1,560,461

7.

Sole Dispositive Power
- 0 -

8.

Shared Dispositive Power
1,560,461

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
1,560,461 shares

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

Not applicable

11.

Percent of Class Represented by Amount in Row (9)
4.78%

12.

Type of Reporting Person (See Instructions)
IN

Item 1.

(a)

Name of Issuer:
The name of the issuer is Ameristar Casinos, Inc. (the “Company”).

(b)

Address of Issuer’s Principal Executive Offices:
The Company’s principal executive offices are located at 3773 Howard Hughes Parkway, Suite 490 South, Las Vegas, Nevada 89169.

Item 2.

(a)

Name of Person(s) Filing:
This statement is filed by:

(i)        Sirios Capital Partners, L.P., a Delaware limited partnership (“SCP I”), with respect to the shares of Common Stock directly owned by it;

(ii)       Sirios Capital Partners II, L.P., a Delaware limited partnership (“SCP II”), with respect to the shares of Common Stock directly owned by it;

(iii)      Sirios Overseas Fund, Ltd., a Cayman Islands company (“SOF”), with respect to the shares of Common Stock directly owned by it;

(iv)      Sirios Focus Partners, L.P., a Cayman Islands exempted limited partnership (“SFP”), with respect to the shares of Common Stock directly owned by it;

(v)       Sirios Capital Management, L.P., a Delaware limited partnership (“SCM”), which serves as investment manager to SCP I, SCP II, SOF, and SFP, with respect to the shares of Common Stock directly owned by SCP I, SCP II, SOF, and SFP;

(vi)      Sirios Associates, L.L.C., a Delaware limited liability company (“SA”), which is the general partner of SCM, with respect to the shares of Common Stock directly owned by SCP I, SCP II, SOF, and SFP; and

(vii)     John F. Brennan, Jr., the sole managing member of SA, with respect to the shares of Common Stock directly owned by SCP I, SCP II, SOF, and SFP.

The foregoing persons are hereinafter sometimes collectively referred to as the “Reporting Persons.” Any disclosures herein with respect to persons other than the Reporting Persons are made on information and belief after making inquiry to the appropriate party.

(b)

Address of Principal Business Office or, if none, Residence:
The address of the business office of SCP I, SCP II, SCM, SA, and Mr. Brennan is One International Place, Boston, Massachusetts 02110-2649.  The address of the registered office of SOF and SFP is c/o Goldman Sachs Administration Services, Gardenia Court, Suite 3307, 45 Market Street, Camana Bay, PO Box 896, KY1-1103, Cayman Islands.

(c)

Citizenship:
SCP I, SCP II, and SCM are limited partnerships organized under the laws of the State of Delaware.  SFP is an exempted limited partnerships organized under the laws of the Cayman Islands.  SOF is a company organized under the laws of the Cayman Islands.  SA is a limited liability company organized under the laws of the State of Delaware.  Mr. Brennan is a United States citizen.

(d)

Title of Class of Securities:
Common Stock, par value $0.01 per share

(e)

CUSIP Number:
03070Q101

Item 3.

If this statement is filed pursuant to §§240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:

(a)

o

Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o).

(b)

o

Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c).

(c)

o

Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c).

(d)

o

Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C 80a-8).

(e)

o

An investment adviser in accordance with §240.13d-1(b)(1)(ii)(E);

(f)

o

An employee benefit plan or endowment fund in accordance with §240.13d-1(b)(1)(ii)(F);

(g)

o

A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);

(h)

o

A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);

(i)

o

A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);

(j)

o

A non-U.S. institution in accordance with § 240.13d–1(b)(1)(ii)(J);

(k)

o

Group, in accordance with § 240.13d–1(b)(1)(ii)(K). If filing as a non-U.S. institution in accordance with

§ 240.13d–1(b)(1)(ii)(J), please specify the type of institution:____________________________

Item 4.

Ownership.

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.

As of December 31, 2011, the Reporting Persons may be deemed to beneficially own 4.78% of the outstanding Common Stock, par value $0.01 per share (“Common Stock”) of the Company.  The percentages used herein are calculated based upon the 32,632,793 shares of Common Stock issued and outstanding, as reflected in the Company’s Form 10-Q for the fiscal quarter ended September 30, 2011, as filed on November 8, 2011.

(a)

Amount beneficially owned:   

A.    Sirios Capital Partners, L.P.:   126,973

B.    Sirios Capital Partners II, L.P.:   482,488

C.    Sirios Overseas Fund, Ltd.:   810,162

D.    Sirios Focus Partners, L.P.:   140,838

E.             Sirios Capital Management, L.P.:   1,560,461

F.     Sirios Associates, L.L.C.:   1,560,461

G.    John F. Brennan, Jr.:   1,560,461

(b)

Percent of class:   

A.    Sirios Capital Partners, L.P.:   0.39%

B.    Sirios Capital Partners II, L.P.:   1.48%

C.    Sirios Overseas Fund, Ltd.:   2.48%

D.    Sirios Focus Partners, L.P.:   0.43%

E.     Sirios Capital Management, L.P.:   4.78%

F.     Sirios Associates, L.L.C.:   4.78%

G.    John F. Brennan, Jr.:   4.78%

(c)

Number of shares as to which the person has:

(i)

Sole power to vote or to direct the vote:   

A.    Sirios Capital Partners, L.P.:      - 0 -

B.    Sirios Capital Partners II, L.P.:   - 0 -

C.    Sirios Overseas Fund, Ltd.:   - 0 -

D.    Sirios Focus Partners, L.P.:   - 0 -

E.     Sirios Capital Management, L.P.:   - 0 -

F.     Sirios Associates, L.L.C.:   - 0 -

G.    John F. Brennan, Jr.:   - 0 -

(ii)

Shared power to vote or to direct the vote:    

A.    Sirios Capital Partners, L.P.:   126,973

B.    Sirios Capital Partners II, L.P.:   482,488

C.    Sirios Overseas Fund, Ltd.:   810,162

D.    Sirios Focus Partners, L.P.:   140,838

E.     Sirios Capital Management, L.P.:   1,560,461

F.     Sirios Associates, L.L.C.:   1,560,461

G.    John F. Brennan, Jr.:   1,560,461

(iii)

Sole power to dispose or to direct the disposition of:   

A.    Sirios Capital Partners, L.P.:      - 0 -

B.    Sirios Capital Partners II, L.P.:   - 0 -

C.    Sirios Overseas Fund, Ltd.:   - 0 -

D.    Sirios Focus Partners, L.P.:   - 0 -

E.     Sirios Capital Management, L.P.:   - 0 -

F.     Sirios Associates, L.L.C.:   - 0 -

G.    John F. Brennan, Jr.:   - 0 -

(iv)

Shared power to dispose or to direct the disposition of:   

A.    Sirios Capital Partners, L.P.:   126,973

B.    Sirios Capital Partners II, L.P.:   482,488

C.    Sirios Overseas Fund, Ltd.:   810,162

D.    Sirios Focus Partners, L.P.:   140,838

E.     Sirios Capital Management, L.P.:   1,560,461

F.     Sirios Associates, L.L.C.:   1,560,461

G.    John F. Brennan, Jr.:   1,560,461

Item 5.

Ownership of Five Percent or Less of a Class.

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following   x.

Item 6.

Ownership of More than Five Percent on Behalf of Another Person.

Not applicable

Item 7.

Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company or Control Person.

Not applicable

Item 8.

Identification and Classification of Members of the Group.

Not applicable

Item 9.

Notice of Dissolution of Group.

Not applicable

Item 10.

Certification.

Each of the Reporting Persons hereby makes the following certification:

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

/s/ John F. Brennan, Jr.

John F. Brennan, Jr., individually, and as sole managing member of Sirios Associates, L.L.C., the general partner of Sirios Capital Management, L.P., the investment manager of Sirios Capital Partners, L.P., Sirios Capital Partners II, L.P., Sirios Overseas Fund, Ltd., and Sirios Focus Partners, L.P.

/s/ John F. Brennan, Jr.

John F. Brennan, Jr., individually, and as sole managing member of Sirios Associates, L.L.C., the general partner of Sirios Capital Management, L.P., the investment manager of Sirios Capital Partners, L.P., Sirios Capital Partners II, L.P., Sirios Overseas Fund, Ltd., and Sirios Focus Partners, L.P.