SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

Amentum Holdings, Inc.

(Name of Issuer)

COMM

(Title of Class of Securities)

023939101

(CUSIP Number)

06/30/2026

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

Rule 13d-1(b)

Rule 13d-1(c)

Rule 13d-1(d)

SCHEDULE 13G

CUSIP Number(s):

023939101

Names of Reporting Persons

PRIMECAP MANAGEMENT CO/CA/

Check the appropriate box if a member of a Group (see instructions)

(a)

(b)

Sec Use Only

Citizenship or Place of Organization

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 12,185,573.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 12,285,504.00
8	Shared Dispositive Power 0.00

Aggregate Amount Beneficially Owned by Each Reporting Person

12,285,504.00

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in row (9)

5.03 %

Type of Reporting Person (See Instructions)

SCHEDULE 13G

Item 1.

(a)

Name of issuer:

Amentum Holdings, Inc.

(b)

Address of issuer's principal executive offices:

4800 WESTFIELDS BOULEVARD, SUITE 400, CHANTILLY, VIRGINIA 20151

Item 2.

(a)

Name of person filing:

PRIMECAP MANAGEMENT CO/CA/

(b)

Address or principal business office or, if none, residence:

177 EAST COLORADO BLVD. 11TH FLOOR PASADENA, California 91105

(c)

Citizenship:

PRIMECAP MANAGEMENT CO/CA/ - UNITED STATES

(d)

Title of class of securities:

COMM

(e)

CUSIP Number(s):

023939101

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)

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

(b)

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

(c)

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

(d)

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

(e)

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

(f)

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

(g)

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

(h)

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

(i)

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)

A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J). If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J),
please specify the type of institution:

(k)

Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).

Item 4.

Ownership

(a)

Amount beneficially owned:

12,285,504

(b)

Percent of class:

5.03 %

(c)

Number of shares as to which the person has:

(i) Sole power to vote or to direct the vote:

PRIMECAP MANAGEMENT CO/CA/ - 12,185,573

(ii) Shared power to vote or to direct the vote:

PRIMECAP MANAGEMENT CO/CA/ - 0

(iii) Sole power to dispose or to direct the disposition of:

PRIMECAP MANAGEMENT CO/CA/ - 12,285,504

(iv) Shared power to dispose or to direct the disposition of:

PRIMECAP MANAGEMENT CO/CA/ - 0

Item 5.

Ownership of 5 Percent or Less of a Class.

Not Applicable

Item 6.

Ownership of more than 5 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.

Certifications:

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and 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, other than activities solely in connection with a nomination under § 240.14a-11.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

PRIMECAP MANAGEMENT CO/CA/

Signature:	JENNIFER OTTOSEN
Name/Title:	CHIEF COMPLIANCE OFFICER
Date:	08/06/2026