If you're trying to decide whether MIAX's growth is real or a bubble about to pop, one data point matters more than any analyst model: who's buying, and why. Ken Griffin's firm sees roughly one out of every three US listed options trades before almost anyone else does. That same firm has owned a piece of MIAX since 2015, added to that position again in 2026, and physically sits on MIAX's trading floor every day making markets.

That's not a headline someone tweeted. That's public record. Here's what it actually means, and what it doesn't.

The business, in one sentence

MIAX runs options, equities, and futures exchanges, and collects a small fee every time a contract trades on them. Options carry 88% of net revenue. Net revenue hit $141.1 million last quarter, up 35% year over year, at a 55% EBITDA margin.

The whole valuation question comes down to one number: does trading volume keep climbing, or does it revert. Three-year analyst consensus currently models MIAX's revenue actually declining roughly 25% a year, on the theory that a meaningful share of the last six years' volume boom was retail and hedging activity that scales with market volatility, and that boom has never been tested by a real calm stretch. That's the tension. Now here's who's betting real money on the answer, and why their bet is worth more than the average investor's.

This isn't a recent bet

Start with the timeline, because it changes the whole read. Citadel Securities didn't discover MIAX this year. In February 2015, MIAX closed an equity rights program that brought in Citadel Securities alongside KCG Americas, Morgan Stanley, Optiver US, Susquehanna Securities, Timber Hill, and Wolverine Trading, the largest options market-making firms in the country, all at once, all taking a piece of the same exchange operator.

That's eleven years of ownership by the time this article publishes. Not a momentum trade. Not a headline chase. A decade-plus infrastructure bet by the firms that actually make markets for a living.

What Citadel Securities sees that you don't

Here's the part worth sitting with. Citadel Securities is the largest on-exchange options market maker in the country, with roughly 30% share of the entire US listed options market. It trades almost a third of every option that changes hands in America. It executes about 35% of all US retail options volume specifically.

That is not a research analyst reading MIAX's earnings release once a quarter. That is a firm watching the actual order flow, in real time, across nearly a third of the market, every single trading day. MIAX's own revenue is a straightforward function of one number: how many options contracts trade, and at what fee, across the exchanges it operates. If anyone is positioned to know whether that number keeps climbing or reverts to where it started, it's the firm pricing and hedging that flow at scale before the rest of the market sees it in a 10-Q.

Griffin's other business, Citadel Securities' equities desk, handles roughly one of every four stock trades in the US. Between the two, very little of what actually moves through American markets happens without this firm seeing it first.

The trading floor detail almost nobody mentions

When MIAX opened its physical options trading floor in Miami in September 2025, three firms showed up to make markets in person: Jane Street, Susquehanna, and Citadel Securities.

That matters more than it sounds. Picture three trading desks on the same floor, shoulder to shoulder, all quoting prices on the same contracts, and one of them also holds a piece of the building they're standing in. Citadel isn't just a passive shareholder collecting a line item on a 13F. It is simultaneously an owner of the exchange and one of the firms physically providing the liquidity that makes the exchange worth trading on. Investor and customer, at the same time, on the same floor.

The buying didn't stop in 2015

Two separate, sophisticated, concentrated investors both added meaningfully to MIAX in 2026, independently of each other.

Citadel Advisors LLC, Griffin's multi-strategy hedge fund and a legally distinct entity from Citadel Securities, added 988,200 shares to its MIAX position in the first quarter of 2026. That's fresh capital, deployed this year, from the portfolio side of Griffin's empire, on top of the market-making side's decade-old stake.

Separately, Horizon Kinetics made Miami International Holdings its single most impactful portfolio move of the second quarter of 2026, and the real story here is sadder, and more interesting, than a simple insider add. Murray Stahl, Horizon Kinetics' co-founder, Chairman, and CEO for over three decades, died suddenly on April 7, 2026. Seventeen days later, on April 24, the firm had a Managing Director, Eric Sites, elected to MIAX's board. Three days after that, Horizon Kinetics' stake had grown to 12.2% of the company.

That is not a legacy position drifting on autopilot while lawyers sort out an estate. That is the surviving leadership, co-CEOs Steven Bregman and Peter Doyle, choosing to deepen the firm's highest-conviction position and take a board seat in the immediate aftermath of losing their founder. The stake was later brought back to roughly 7.8-8% through a routine pro-rata in-kind distribution of shares to the firm's own fund clients in May, not a sale and not a change of heart.

Stahl's own framing, built over decades running the firm, is worth quoting directly, because it's a different argument than Griffin's: exchanges are close to perpetual call options, among the oldest continually operating public companies in the world, businesses that "don't fail, dwindle away, or get displaced." That was never a bet on one quarter's options volume. It's a thesis his successors chose to lean into harder, not walk back, at the exact moment it would have been easiest to play it safe.

Two firms, two different reasons, both adding in the same year, one of them through a leadership transition that would have given most firms an excuse to go quiet instead. Both file 13Fs quarterly, so this isn't the last time you'll be able to check whether they kept adding.

What this doesn't tell you

Here's the honest caveat, because owning this stock because "Ken Griffin owns it too" is exactly the kind of lazy thinking that gets punished.

Market makers taking equity stakes in exchanges isn't purely a bet that the stock goes up. It's also a strategic move to reduce dependence on incumbent venues, secure favorable access and fee treatment, and have a seat at the table when a challenger exchange sets its rules. Citadel, Susquehanna, and Wolverine collectively hold pieces of more than one exchange for exactly this reason. Owning equity in the venue you trade on is a hedge against being at the mercy of Cboe, ICE, or Nasdaq's fee schedules, not only a conviction call on MIAX's stock price specifically.

It's also worth naming the concentration risk this creates. A meaningful share of MIAX's own trading volume runs through a small number of market-making giants. If Citadel, Jane Street, or Susquehanna ever meaningfully shifted their liquidity elsewhere, MIAX would feel it directly. The same firms whose ownership signals conviction are also the firms whose behavior the whole exchange depends on.

Five scenarios

Two forces are pulling on this stock in opposite directions: analyst models that assume the volume boom partially reverses, and the people with the best seat in the house buying more anyway. Weigh both, and here's where that lands.

Super Bull. Institutional and AI-driven algorithmic volume keeps expanding as a share of total flow, providing a structural floor that isn't tied to retail sentiment, while retail 0DTE participation also holds. MIAX keeps taking share from incumbents. Net revenue compounds 25%/yr for five years, margin expands past 60%, the market re-rates the stock toward 25x EBITDA as a structural compounder. Five-year annualized return: roughly 35%.

Bull. Volume holds near current elevated levels. The growing institutional/algorithmic share cushions the business against a pure retail slowdown, and MIAX's largest owners keep adding. Net revenue grows 12-15%/yr, margins keep expanding, multiple re-rates modestly to 18-19x. Annualized return: roughly 15%.

Base case. Some real mean reversion happens on the retail/volatility-driven slice of volume, roughly consistent with what analyst models already project, but it's partially offset by a growing structural floor of institutional and algorithmic flow, and by the fact that the exchange's largest outside owners, the ones with the best information, are adding rather than trimming. Net revenue compounds closer to 9%/yr over five years, well below the 35% currently being posted but well above a straight-line extrapolation of the bear consensus. Margin expansion slows without volume growth doing the heavy lifting, multiple holds roughly flat near 16x. Annualized return: roughly 9%.

Bear. The 20-25% revenue contraction analysts already model plays out largely as feared, and the institutional/AI volume floor turns out smaller than hoped. Multiple compresses toward 10-12x as the market repriced this from growth story to cyclical exchange. Annualized return: roughly -6%.

Super bear. A structural volatility drought combines with one or more of the top market-making owners quietly shifting liquidity to a competing venue, the exact concentration risk named above. Revenue keeps contracting past year two, multiple falls toward 8x. Price target: roughly 50% below today.

The takeaway

Ken Griffin's firm didn't stumble into MIAX. It's owned a piece since 2015, added more in 2026, and sits on the trading floor making markets every day, with a real-time view of order flow that no outside analyst has. Horizon Kinetics added even more the same year, under new leadership after Murray Stahl's death, for a completely different reason than Griffin's: exchanges as near-permanent infrastructure, a thesis the firm chose to double down on rather than quietly walk back.

Neither of those facts proves the bull case. Owning equity in the venue you trade on is also just good business, not only conviction. But when two independent, long-horizon, well-informed investors both add in the same year, on top of an eleven-year-old strategic stake, that's a real signal worth weighing against the revenue-decline models, not a reason to ignore them. Most people staring at MIAX's valuation only see the 25% decline forecast. You now know who's on the other side of that trade, and why.

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