Amentum stake: PRIMECAP reports 5.03% ownership

PRIMECAP MANAGEMENT CO/CA/ has disclosed a significant ownership position in Amentum Holdings, Inc. The institutional investor reports beneficial ownership of 12,285,504 COMM shares, representing 5.03% of the class.

Rhea-AI Filing Summary

PRIMECAP MANAGEMENT CO/CA/ has disclosed a significant ownership position in Amentum Holdings, Inc. The institutional investor reports beneficial ownership of 12,285,504 COMM shares, representing 5.03% of the class.

PRIMECAP holds sole voting power over 12,185,573 shares and sole dispositive power over all 12,285,504 shares, with no shared voting or dispositive authority. The filing identifies PRIMECAP as organized in the United States, with its principal office in Pasadena, California, and Amentum’s principal executive offices in Chantilly, Virginia.

Key Figures

Beneficial ownership: 12,285,504 shares Ownership percentage: 5.03% Sole voting power: 12,185,573 shares +3 more

6 metrics

Beneficial ownership 12,285,504 shares Amount beneficially owned in Amentum COMM shares

Ownership percentage 5.03% Percent of Amentum COMM class beneficially owned

Sole voting power 12,185,573 shares Shares over which PRIMECAP has sole power to vote

Shared voting power 0 shares Shares over which PRIMECAP has shared power to vote

Sole dispositive power 12,285,504 shares Shares over which PRIMECAP has sole power to dispose

Shared dispositive power 0 shares Shares over which PRIMECAP has shared power to dispose

Key Terms

beneficially owned, Sole Voting Power, Sole Dispositive Power, Percent of class

4 terms

beneficially owned financial

"Amount beneficially owned: 12,285,504"

Beneficially owned describes securities or assets where a person has the economic rights and control—such as the right to receive dividends and to direct voting—even if legal title is held in another name. Think of it like having the keys and using a car that’s registered to someone else: you get the benefits and make decisions. Investors care because beneficial ownership reveals who truly controls value and voting power, affecting corporate decisions and takeover dynamics.

Sole Voting Power financial

"5 | Sole Voting Power 12,185,573.00"

Sole voting power is the exclusive right to cast votes attached to a shareholder’s stock without needing approval from anyone else. Like holding the only remote control for a TV, it lets that holder decide corporate matters such as board members, mergers, and policy changes, making it important to investors because it concentrates control and can strongly influence a company’s strategy and the value of its shares.

Sole Dispositive Power financial

"7 | Sole Dispositive Power 12,285,504.00"

Sole dispositive power is the exclusive legal authority to decide what happens to a security — for example, whether to sell, transfer, or retain shares — without needing anyone else’s permission. Investors care because it signals who truly controls the economic outcome of an investment: like holding the only key to a safe, the holder can realize gains or losses and may trigger regulatory reporting, insider rules, or influence over corporate ownership.

Percent of class financial

"(b) | Percent of class: 5.03 %"

Percent of class is the portion of a specific category of securities—such as a company’s common shares, preferred shares, or a bond series—that takes part in or approves a corporate action (vote, consent, tender, etc.). Investors watch this number because it reveals how much support or opposition exists within that particular shareholder group; like counting how many members of a club back a proposal, it can determine whether a plan passes or how influence is distributed.

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

What percentage of Amentum Holdings, Inc. (AMTM) does PRIMECAP Management report owning?

PRIMECAP Management reports beneficial ownership of 5.03% of Amentum Holdings, Inc.’s COMM shares, based on 12,285,504 shares. This level of ownership makes PRIMECAP a significant institutional holder in the company.

How many Amentum (AMTM) shares does PRIMECAP Management beneficially own?

PRIMECAP Management beneficially owns 12,285,504 Amentum COMM shares. This stake corresponds to 5.03% of the outstanding class, according to the ownership information disclosed in the Schedule 13G filing.

What voting power does PRIMECAP Management have over Amentum (AMTM) shares?

PRIMECAP Management has sole voting power over 12,185,573 Amentum shares and no shared voting power. This means PRIMECAP alone can vote or direct the vote for those shares without another party’s consent.

What dispositive power does PRIMECAP hold over its Amentum (AMTM) position?

PRIMECAP holds sole dispositive power over 12,285,504 Amentum shares and no shared dispositive power. Sole dispositive power allows PRIMECAP to decide independently whether to sell or otherwise dispose of these shares.

Who signed the Schedule 13G for PRIMECAP Management regarding Amentum (AMTM)?

The Schedule 13G was signed by Jennifer Ottosen, identified as Chief Compliance Officer. The signature confirms the reported ownership details on behalf of PRIMECAP Management as of the filing date.

Where are Amentum Holdings, Inc. (AMTM) and PRIMECAP Management based?

Amentum’s principal executive offices are at 4800 Westfields Boulevard, Suite 400, Chantilly, Virginia 20151. PRIMECAP Management’s principal business office is at 177 East Colorado Blvd., 11th Floor, Pasadena, California 91105.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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Available on EDGAR 08/06/2026 - 05:17 PM Accepted by SEC EDGAR 08/06/2026 - 05:16 PM Learn about SEC filing dates

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)

schemaVersion:

SCHEDULE 13G

CUSIP Number(s): 023939101

1

Names of Reporting Persons

PRIMECAP MANAGEMENT CO/CA/

2

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

(a)
(b)

3

Sec Use Only

4

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

9

Aggregate Amount Beneficially Owned by Each Reporting Person

12,285,504.00

10

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

11

Percent of class represented by amount in row (9)

5.03 %

12

Type of Reporting Person (See Instructions)

IA

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