Summary of William Blair Investment Management LLC portfolio changes for the quarter ending 2026-03-31, including a new Birkenstock position and increased stakes in Gold Fields, Millicom, Sterling Infrastructure, and TotalEnergies.

William Blair Investment Management, LLC: 13F Portfolio Moves Tracked

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Quarter Ending 2026-03-31

During the quarter ending 2026-03-31, William Blair Investment Management, LLC made notable adjustments across its equity portfolio, with a clear tilt toward adding to existing positions rather than establishing new ones. The most significant new commitment was a fresh stake in Birkenstock Holding PLC (BIRK), valued at $92,276,654.

Several holdings saw dramatic percentage increases. Gold Fields Ltd (GFI) surged 4052%, growing from $1,121,014 to $46,542,125, while Millicom Intl Cellular S A rose 1418%, moving from $6,939,702 to $105,336,264. Sterling Infrastructure Inc (STRL) also stood out with a 943% increase, expanding from $12,633,212 to $131,719,671, and TotalEnergies SE (TTE) climbed 642%, from $10,753,323 to $79,795,328.

Other meaningful additions included C H Robinson Worldwide In, up 146% to $172,988,144; Grand Canyon Ed Inc, up 121% to $166,584,172; StepStone Group Inc (STEP), up 112% to $91,358,842; and Alignment Healthcare Inc (ALHC), up 102% to $91,881,499. Mid-range increases were recorded in Reliance Inc (RS), Regal Rexnord Corporation (RRX), Clean Harbors Inc (CLH), Halozyme Therapeutics Inc (HALO), Nextpower Inc (NXT), and F5 Inc (FFIV), each rising between 32% and 65%.

Smaller but still notable additions appeared in Twist Bioscience Corp (TWST), Cava Group Inc (CAVA), Lattice Semiconductor Corp (LSCC), Primo Brands Corporation (PRMB), and Linde PLC (LIN), ranging from 15% to 36%. This report reflects positioning data only and should not be interpreted as investment advice.

Related ticker coverage

This page follows William Blair Investment Management, LLC’s institutional portfolio moves as disclosed in its quarterly SEC Form 13F filings. Rather than a one-off snapshot, it’s meant to be a running record: each quarter we add the fund’s newly reported buys and sells here, so you can see how its US equity holdings evolve filing by filing. Form 13F data reflects a point-in-time snapshot of holdings at quarter end, not the specific days trades were placed, so all activity below is described as occurring ‘during the quarter’ rather than on a precise date. Nothing here is investment advice or a recommendation to buy or sell any security.

William Blair Investment Management, LLC is a Chicago-based institutional asset manager whose 13F filings are watched by those tracking how established growth-oriented shops are positioning across sectors, from healthcare to industrials to semiconductors.

Quarter Ending 2026-03-31: New Positions Opened

During the quarter ending 2026-03-31, William Blair Investment Management, LLC reported a wave of new positions across a wide range of industries. The largest by dollar value was AstraZeneca PLC (AZN), a new stake worth roughly $377.3 million. Also new this quarter was a position in Akamai Technologies Inc (AKAM) worth about $185.9 million, followed by BorgWarner Inc (BWA) at roughly $156.5 million and Amphenol Corp at approximately $136.1 million.

Additional new positions opened during the quarter include Littelfuse Inc ($103.4 million), Synaptics Inc (SYNA, $111.1 million), Power Integrations Inc (POWI, $121.9 million), National Bk Hldgs Corp ($96.9 million), Kennametal Inc (KMT, $89.7 million), Guardant Health Inc (GH, $87.9 million), Casey’s Gen Stores Inc ($86.0 million), Cullen Frost Bankers Inc (CFR, $83.9 million), Texas Instrs Inc ($82.4 million), Cytokinetics Inc (CYTK, $71.7 million), API Group Corp (APG, $70.5 million), Southern Copper Corp (SCCO, $63.3 million), Saia Inc (SAIA, $59.8 million), Terex Corp New (TEX, $50.4 million), Teva Pharmaceutical Inds Ltd ($42.9 million), and Vista Energy S.A.B. de C.V. (VIST, $42.0 million).

What to Watch Next Quarter

No exits, trims, or additions to existing holdings were disclosed for this filing period beyond the new positions listed above. As William Blair Investment Management, LLC’s next 13F becomes available, this profile will be updated with fresh moves, including any positions increased, reduced, or closed out entirely, so this page can continue to serve as a single reference point for the fund’s reported activity over time.

Source: SEC Form 13F filings. This page updates every quarter.

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