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

Rhea-AI Filing Summary

William Blair Investment Management, LLC filed Amendment No. 1 to Schedule 13G reporting a passive ownership position in Montrose Environmental Group (MEG).

• Shares owned: 2,317,660 common shares
• Ownership percentage: 6.8 % of outstanding stock
• Sole voting power: 1,924,734 shares; shared voting: 0
• Sole dispositive power: 2,317,660 shares; shared dispositive: 0
• Trigger date: 30 Jun 2025

The filer, organized in Delaware and classified as an investment adviser under Rule 13d-1(b)(1)(ii)(E), certifies that the securities were acquired in the ordinary course of business and not to influence control of the issuer.

Positive

• Institutional ownership rises to 6.8 %, indicating confidence from a respected investment adviser
• All shares are held passively, reducing risk of activist disruption while adding liquidity

Negative

• None.

Insights

Securities Analyst

TL;DR: 6.8 % passive stake by William Blair adds institutional support; limited direct impact on governance.

The disclosure shows William Blair now holds 2.32 million MEG shares, crossing the 5 % threshold that requires a Schedule 13G. As the filing is under Rule 13d-1(b), the firm states it is a passive investment adviser with no control intent. Institutional ownership of nearly 7 % can improve float stability and liquidity, and may be perceived positively by other investors as a sign of confidence in MEG’s prospects. However, because no activist motives are declared and voting power is below 10 %, the event is not expected to materially change corporate strategy or board composition in the near term.

Corporate Governance Expert

TL;DR: Filing is routine; governance implications minimal given passive intent.

Form 13G/A reaffirms that William Blair’s position is strictly passive. Sole voting authority over 1.92 million shares gives the adviser some influence in routine matters but remains well below levels that would pressure management. No group formation, control purpose, or coordinated action is indicated. Therefore, the filing is largely informational, satisfying SEC disclosure rules without signaling any governance challenge.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 1)

Montrose Environmental Group, Inc.

(Name of Issuer)

Common Stock

(Title of Class of Securities)

615111101

(CUSIP Number)

06/30/2025

(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 1,924,734.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 2,317,660.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

2,317,660.00

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)

6.8 %

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: 2,317,660
(b)	Percent of class: 6.8%
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: 1,924,734
	(ii) Shared power to vote or to direct the vote: 0
	(iii) Sole power to dispose or to direct the disposition of: 2,317,660
	(iv) Shared power to dispose or to direct the disposition of: 0
Item 5.	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:	07/30/2025

FAQ

How many Montrose Environmental (MEG) shares does William Blair Investment Management own?

The firm reports beneficial ownership of 2,317,660 common shares.

What percentage of MEG’s outstanding stock is represented by William Blair’s stake?

William Blair holds 6.8 % of the company’s common shares.

Does William Blair intend to influence Montrose Environmental’s control or strategy?

No. The Schedule 13G filing states the shares were acquired in the ordinary course of business with no control intent.

What type of filer is William Blair Investment Management under SEC rules?

It is classified as an investment adviser pursuant to Rule 13d-1(b)(1)(ii)(E).

When did the ownership threshold requiring this filing occur?

The trigger event date disclosed is June 30, 2025.