Millennium Management, the New York-based multi-strategy hedge fund founded by Israel Englander in 1989, and Abu Dhabi's Mubadala Investment Company, a state-owned sovereign investor, both increased their holdings of BlackRock's iShares Bitcoin Trust (IBIT) in the fourth quarter, according to a November 6 research brief from Galaxy, a digital-assets financial services firm. Their paths diverged only in early 2026: Millennium's stake has fallen roughly 72% from its December level, while Mubadala's share count was unchanged in the second quarter.

The brief builds on an October report comparing Form 13F filings as of September 30, 2025, and June 30, 2026 — the quarterly SEC disclosures institutional investment managers must file once they oversee $100 million or more in US-listed securities. The endpoints are deliberate: September 30, 2025, was the last quarter-end before Bitcoin's October peak, and the cycle low arrived on June 30, 2026. Across that span, Galaxy says, ETF holdings reported by 13F filers fell 14%. The firm names Millennium among nine multi-strategy, macro and bank trading desks that together cut 68,000 BTC of exposure, and counts Mubadala among the wealth platforms and sovereign funds that bought.

At those two dates, the filings bear the argument out: Millennium's ordinary shares in IBIT fell about 53%, while Mubadala's rose about 69%. The December and March filings in between, however, change the order of events.

In the December quarter, with IBIT's quarter-end mark down by about a quarter, both managers bought. Millennium's December count was the largest it had reported at any quarter-end since IBIT launched in January 2024, in the first wave of spot bitcoin ETFs approved for US trading.

The paths parted in the March quarter. Millennium sold 44% of its position while Mubadala kept adding — the only three months in this comparison in which the two moved in opposite directions. In the June quarter, Millennium cut again and Mubadala held still: its March and June filings both show exactly 14,721,917 shares.

Measured from December rather than September, Millennium's reduction comes to roughly 72%, not 53%. Galaxy's endpoints understate how much the fund sold, and they leave out that it was buying alongside the sovereign investor three months earlier.

Notes on the data: positions are based on ordinary shares reported in SEC Form 13F filings — for Millennium and Mubadala as of September 30, 2025; for Millennium as of December 31, 2025 (Mubadala's December 2025 count as reported by CoinDesk from its 13F); for Millennium and Mubadala as of March 31, 2026; and for Millennium and Mubadala as of June 30, 2026. The IBIT mark is the reported position value divided by shares held at each quarter-end, rounded to whole dollars.

Mubadala's stake grew while its value shrank

The IBIT mark explains a result that looks contradictory in Mubadala's filings. Its position was reported at $567 million in September 2025 and $490 million in June 2026 — about 14% less — even though it held 69% more shares. Each share was marked about 49% lower over that period, matching the 49% decline in Bitcoin that Galaxy cites.

Those marks are quarter-end prices, not purchase costs. The filings do not show what Mubadala paid for the roughly 6 million shares it added, and the positions are fund shares rather than bitcoin held directly — a distinction Coindoo examined in what owning a crypto ETF actually gives investors. For tracking whether a position grew, the share count is the cleaner measure.

The same arithmetic separates selling from price on Millennium's side. Most of the drop in its reported IBIT value between December 31 and March 31, calculated from its December and March 13F filings, came from shares it no longer held rather than from the lower mark.

A fund can trade Bitcoin ETFs without a view on Bitcoin

Building the largest stake it had ever reported into year-end, then unwinding most of it within six months, fits a trade that does not depend on Bitcoin's direction. In a basis trade, as outlined in a CME Group explainer, a fund buys spot exposure such as ETF shares and shorts Bitcoin futures, collecting the gap between the two prices. When the gap narrows, the trade is closed by selling the ETF and buying back the futures.

Form 13F shows only the first half of that transaction. The SEC's 13F guidance excludes short positions, and Bitcoin futures, which trade under CFTC oversight, are not 13F securities. A basis unwind therefore appears in the filings as plain ETF selling. The IBIT options Millennium did disclose were small next to its shares: from September 2025 through March 2026, its call and put rows each covered fewer than 1.5 million underlying shares. Mubadala's June filing lists a single IBIT row with no options attached.

Aggregate futures data covers part of what the filings omit. Citing the CFTC's Traders in Financial Futures report, Galaxy says leveraged funds' net short in CME Bitcoin futures — the futures leg of ETF basis trades — shrank from 71,400 BTC in the week of the October 2025 high to 30,600 BTC by late October 2026. Over the 30 days before the high, the front-month basis averaged 7.9% annualized, 3.9 points above three-month Treasury bills. In the 30 days to October 21, it averaged 4.8%, just 0.6 points above bills. Galaxy concludes that this shrinking margin is why multi-strategy desks left.

That is an industry-wide explanation, and Millennium's timing is consistent with it. No filing ties Millennium's sales to a futures short, however. The same limit applies on the buying side: Mubadala's single share row shows a larger holding, not how long it plans to keep it.

What the September filings can settle

Third-quarter 13F reports are due November 16 — the usual 45-day deadline after the September 30 quarter-end lands on a Saturday — and cover holdings on September 30, 2026. They are the first filings to show who held ETF shares after the June low. Galaxy, which told clients in October that it believes Bitcoin's bottom is in, reads the 87,000 BTC the ETFs have added since that low as directional buying rather than carry, because leveraged funds' futures shorts fell over the same stretch. The new filings put names to part of that demand. Two counts from this comparison are the first to check.