[SCHEDULE 13G/A] Montrose Environmental Group, Inc. SEC Filing

Rhea-AI Filing Summary

William Blair Investment Management, LLC filed Amendment No. 2 to Schedule 13G for Montrose Environmental Group, Inc., reporting beneficial ownership of 1,315,018 shares of Common Stock, or 3.7% of the class, tied to the event date 09/30/2025.

The filer reports sole voting power over 1,052,190 shares and sole dispositive power over 1,315,018 shares, with no shared voting or dispositive power. The reporting person is an investment adviser and certifies the securities were acquired and are held in the ordinary course and not to change or influence control.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 2)

Montrose Environmental Group, Inc.

(Name of Issuer)

Common Stock

(Title of Class of Securities)

615111101

(CUSIP Number)

09/30/2025

(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 No. 615111101

1

Names of Reporting Persons

WILLIAM BLAIR INVESTMENT MANAGEMENT, LLC

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

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 1,052,190.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 1,315,018.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

1,315,018.00

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)

3.7 %

12

Type of Reporting Person (See Instructions)

IA

SCHEDULE 13G

Item 1.
(a)	Name of issuer: Montrose Environmental Group, Inc.
(b)	Address of issuer's principal executive offices: 4 PARK PLAZA, SUITE 790, 4 PARK PLAZA, SUITE 790, IRVINE, CALIFORNIA, 92614.
Item 2.
(a)	Name of person filing: William Blair Investment Management, LLC
(b)	Address or principal business office or, if none, residence: 150 North Riverside Plaza, Chicago, IL 60606
(c)	Citizenship: Delaware
(d)	Title of class of securities: Common Stock
(e)	CUSIP No.: 615111101
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: 1,315,018
(b)	Percent of class: 3.7%
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: 1,052,190
	(ii) Shared power to vote or to direct the vote: 0
	(iii) Sole power to dispose or to direct the disposition of: 1,315,018
	(iv) Shared power to dispose or to direct the disposition of: 0
Item 5.	Ownership of 5 Percent or Less of a Class.
	Ownership of 5 percent or less of a class
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.

WILLIAM BLAIR INVESTMENT MANAGEMENT, LLC

Signature:	Walter R. Randall, Jr.
Name/Title:	Chief Compliance Officer
Date:	11/12/2025

FAQ

What did William Blair disclose in MEG’s Schedule 13G/A?

It reported beneficial ownership of 1,315,018 shares of Montrose Environmental Group common stock, representing 3.7% of the class.

What voting and dispositive powers did William Blair report for MEG (NYSE: MEG)?

It has sole voting power over 1,052,190 shares and sole dispositive power over 1,315,018 shares, with no shared powers.

What is the event date associated with this MEG 13G/A filing?

The event date is 09/30/2025.

What percentage of MEG’s common stock does this stake represent?

The filing states ownership of 3.7% of the class.

How is the reporting person classified in the filing?

The filer is an investment adviser under §240.13d-1(b)(1)(ii)(E).

Does the filing indicate ownership of 5% or less of MEG’s class?

Yes. Item 5 indicates ownership of 5 percent or less of the class.

Who signed the filing and when?

Signed by Walter R. Randall, Jr., Chief Compliance Officer, on 11/12/2025.