Yes. Most ETFs are open-end funds in the legal sense, classified as open-end investment companies under the Investment Company Act of 1940 and regulated in the same category as traditional mutual funds. What throws people is that the label describes the fund’s legal structure, not how you actually buy, sell, or hold it. An ETF’s shares are redeemable in principle, which is what “open-end” requires, but only a small group of large institutions can redeem directly with the fund. Everyone else trades on a stock exchange, which is why an ETF feels nothing like a mutual fund even though the two sit under the same statute.
What “Open-End” Means Under the 1940 Act
The Investment Company Act of 1940 sorts management companies into two buckets. An open-end company issues redeemable securities: investors can return shares to the fund and receive cash based on net asset value. A closed-end company issues a fixed number of shares that trade among investors on an exchange, with no direct redemption right.
ETFs land on the open-end side because their shares are technically redeemable. The wrinkle is who can redeem them. Only a handful of large institutional firms, called Authorized Participants, hold contracts allowing direct redemption with the fund. Retail investors and everyone else buy and sell on the exchange, which is why the day-to-day experience of owning an ETF resembles owning a stock more than owning a mutual fund.
The legal classification carries real weight. Both ETFs and mutual funds inherit the same investor protections built into the 1940 Act: limits on leverage, daily valuation, restrictions on transactions with affiliates, and mandatory disclosure. The SEC enforces those rules identically across both structures.
How ETFs Differ From Mutual Funds Despite the Same Label
A traditional mutual fund has a direct relationship with every shareholder. Buy shares and the fund creates new ones with your cash, then uses that cash to purchase securities. Sell shares and the fund liquidates enough holdings to pay you back, then cancels your shares. Every order that day, buy or sell, executes at a single price: the net asset value calculated after the market closes, typically around 4:00 p.m. Eastern Time.
That direct model creates operational friction. A wave of redemptions on a bad day forces the manager to sell securities into a falling market, generating transaction costs and realized capital gains that get passed to the shareholders who stayed. The actions of one group of investors impose costs on the rest.
ETFs bolt a completely different trading system on top of the open-end legal frame. Once ETF shares exist, they trade on exchanges like NYSE Arca or Nasdaq throughout the 9:30 a.m. to 4:00 p.m. session. When you buy an ETF, you’re buying from another investor or a market maker on the exchange, not from the fund. The fund’s portfolio doesn’t move when retail shares change hands.
Because ETF shares trade continuously, their market price fluctuates second by second based on supply and demand. That price can drift slightly above NAV (a premium) or below it (a discount). For large, liquid ETFs tracking major indexes, the gap is usually a fraction of a penny. For niche or thinly traded funds, it can be wider.
In 2019, the SEC adopted Rule 6c-11, sometimes called the “ETF Rule,” which standardized the regulatory framework for open-end ETFs. Before it, every new ETF needed its own individual exemptive order from the SEC, a slow process that produced inconsistencies between funds. Rule 6c-11 replaced those piecemeal orders with a single set of conditions any qualifying open-end ETF can operate under.
The Authorized Participant Mechanism
What keeps an ETF’s market price aligned with its underlying value is the Authorized Participant system. APs are large financial institutions, typically major broker-dealers, that hold contracts with ETF providers letting them create and redeem shares directly with the fund. They transact in large standardized blocks called creation units, usually 50,000 shares or more.
When the ETF’s market price drifts above NAV, an AP delivers a basket of the underlying securities to the fund and receives new ETF shares in return, then sells those shares on the exchange for a profit. That new supply pushes the market price back toward NAV. When the market price falls below NAV, the AP buys cheap shares on the exchange, hands them back to the fund for redemption, and receives the more valuable underlying securities. Pulling shares out of circulation nudges the price back up.
The arbitrage runs continuously during trading hours. APs aren’t doing this for the fund’s benefit; they do it because every price gap is a trading opportunity. To make the arbitrage possible, ETFs publish the exact basket of securities APs must deliver or receive each day. Without that transparency, no one could tell whether an arbitrage trade would pay off.
Why the Structure Produces a Tax Advantage
The AP mechanism produces what is probably the single biggest practical benefit of the ETF’s open-end structure. When a mutual fund sells securities to meet redemptions, it realizes capital gains and distributes them to every shareholder in the fund, whether or not they personally sold anything. In 2022, when the S&P 500 fell more than 18%, over 42% of active mutual funds still distributed capital gains averaging around 5% of NAV.
ETFs mostly avoid this because AP redemptions happen in kind. Instead of selling securities to raise cash, the fund hands over the actual securities. Section 852(b)(6) of the Internal Revenue Code exempts these in-kind distributions from triggering capital gains at the fund level. Appreciated positions leave the fund without a taxable event, and shareholders who stay put owe nothing.
Managers have learned to work this mechanism deliberately through what the industry calls “heartbeat trades.” An AP creates new ETF shares by delivering securities, then redeems a similar quantity a few days later. On the redemption side, the fund fills the basket with its most appreciated, lowest-basis holdings, purging embedded gains without a tax bill for shareholders. Research shows roughly 26% of ETFs have used heartbeat trades since 2012, typically about twice a year per fund, usually timed around index rebalancing when portfolio turnover creates the most gains to flush out.
Rule 6c-11 made this easier by simplifying the “custom basket” process, allowing the redemption basket to hold only the specific appreciated securities leaving the fund rather than a pro-rata slice of the whole portfolio.
The ETFs That Are Not Open-End Funds
A small but notable group of ETFs are not organized as open-end funds. Several of the largest and most heavily traded ETFs in the world are structured as unit investment trusts (UITs), including the SPDR S&P 500 ETF (SPY), the SPDR S&P MidCap 400 ETF, and the SPDR Dow Jones Industrial Average ETF.
The operational differences matter if you own one of these:
- UITs cannot immediately reinvest dividends received from underlying stocks; they must hold them in cash until the fund’s distribution date, creating a slight drag on returns.
- UITs must fully replicate the index they track, owning every constituent, while open-end ETFs can sample or optimize.
- UITs cannot lend their securities to generate income; open-end ETFs can.
- UITs cannot hold derivatives; open-end ETFs can use futures, options, and swaps within limits.
Rule 6c-11 does not cover UIT-structured ETFs, which continue to operate under their original exemptive orders. The rule also excludes leveraged and inverse ETFs, which seek to multiply or invert daily index returns. For most ETFs launched in recent years, though, the open-end fund structure is the default.
When the Open-End Promise Strains
The AP arbitrage system works reliably in normal markets but can strain under acute stress. During the March 2020 turmoil, investment-grade and high-yield bond ETFs traded at discounts to NAV ranging from 6% to 10% in the worst cases. The median discount for investment-grade bond ETFs hit 2.0% in the U.S. and 5.4% in Europe, far beyond the fraction-of-a-penny deviations investors normally see.
The dislocation happens when APs pull back. Their willingness to arbitrage depends on their own balance sheet capacity and on the liquidity of the underlying securities. Research on the March 2020 episode found the normal arbitrage response to a one-percentage-point premium dropped by 0.52 percentage points during the crisis, with weaker-capitalized APs pulling back the most. ETFs holding less liquid bonds were affected most heavily.
The dislocations were temporary, lasting days rather than weeks, and the AP mechanism eventually reasserted itself. Still, investors who needed to sell a bond ETF at the worst moment received prices meaningfully below what the underlying bonds were worth. The open-end structure’s promise that price tracks value has a real-world asterisk during panics, and it applies most sharply to funds holding illiquid assets.