Prime money market funds are short-term investment funds that hold corporate commercial paper, bank certificates of deposit, and repurchase agreements, and the income they pay out is taxed as ordinary income at your federal marginal rate. They sit under SEC Rule 2a-7, which controls what the fund can buy, how liquid it must stay, and how its share price is calculated. Because the portfolio is private-sector debt rather than Treasuries, these funds carry slightly more credit risk than government money market funds, and the rules you deal with as an investor depend on whether you buy a retail or an institutional share class.
What a Prime Fund Actually Holds
Rule 2a-7 limits a prime fund to short-term, U.S. dollar-denominated debt that the fund’s board has determined poses minimal credit risk. In practice the portfolio is built from commercial paper issued by large corporations, certificates of deposit from domestic and foreign banks, and repurchase agreements backed by high-quality collateral.1eCFR. 17 CFR 270.2a-7 – Money Market Funds
Three constraints keep the portfolio conservative. No security can have a remaining maturity longer than 397 days when the fund buys it. The fund’s dollar-weighted average maturity cannot exceed 60 days, which keeps the holdings closely tied to current interest rates. And the fund generally cannot put more than 5% of assets into any single issuer, with a narrow three-business-day exception allowing up to 25% in one issuer right after purchase.1eCFR. 17 CFR 270.2a-7 – Money Market Funds
Rule 2a-7 also forces a liquidity cushion. At least 25% of assets must be daily liquid assets (cash or securities maturing within one business day), and at least 50% must be weekly liquid assets convertible to cash within five business days.1eCFR. 17 CFR 270.2a-7 – Money Market Funds That buffer is what lets the fund meet redemptions without dumping longer-dated holdings into a weak market.
Credit judgments are made inside the fund. The SEC removed references to external credit ratings from Rule 2a-7 in 2015, so the fund’s board (usually through the investment adviser) has to independently conclude that each security presents minimal credit risks and keep monitoring that conclusion as market conditions change.2U.S. Securities and Exchange Commission. SEC Removes References to Credit Ratings in Money Market Fund Rule
Retail Versus Institutional Share Classes
The SEC divides prime funds into two categories based on who owns the shares. A retail prime fund has policies reasonably designed to limit all beneficial owners to natural persons.1eCFR. 17 CFR 270.2a-7 – Money Market Funds Individual human beings qualify, and SEC staff guidance has clarified that the estate of a natural person and defined contribution plan participants with investment power over their shares also qualify.3U.S. Securities and Exchange Commission. 2014 Money Market Fund Reform Frequently Asked Questions A sole proprietorship or single-member LLC that opens an account in the entity’s name rather than the individual’s name may not satisfy the natural-person requirement, because the fund has to look at beneficial ownership.
An institutional prime fund is any prime fund that doesn’t impose that natural-person limit. Corporations, pension plans, endowments, and government entities invest through institutional classes.
The biggest practical difference between the two is how the share price is calculated.
Stable NAV for Retail Shares
Retail prime funds use a stable net asset value of $1.00 per share. They’re allowed to value their portfolios using amortized cost and round the share price to the nearest penny.1eCFR. 17 CFR 270.2a-7 – Money Market Funds As a retail investor, you almost always transact at exactly $1.00, and buying or selling doesn’t trigger a capital gain or loss.
Floating NAV for Institutional Shares
Institutional prime funds use a floating NAV. The price is marked to market and carried to four decimal places, so you may see $1.0002 or $0.9998 on any given day.1eCFR. 17 CFR 270.2a-7 – Money Market Funds Every redemption can produce a small gain or loss against your cost basis, which creates tax recordkeeping that retail investors never see.
Liquidity Fees Under the 2023 Reforms
The SEC overhauled the redemption framework in 2023. The old system allowed a fund to temporarily halt redemptions (a “gate”) if weekly liquid assets dropped below 30%. That gate mechanism is gone. No prime fund can block withdrawals through Rule 2a-7 anymore.4U.S. Securities and Exchange Commission. SEC Adopts Money Market Fund Reforms and Amendments to Form PF Reporting
In its place, the rule creates two kinds of liquidity fees:
- A mandatory fee that an institutional prime fund must impose when daily net redemptions exceed 5% of the fund’s net assets. The fee must reflect the fund’s good-faith estimate of the cost of selling securities to meet those redemptions. If the fund cannot make that estimate, a default 1% fee applies.5Federal Register. Money Market Fund Reforms – Form PF Reporting Requirements for Large Liquidity Fund Advisers – Technical Amendments to Form N-CSR and Form N-1A
- A discretionary fee that the board of any non-government money market fund can impose whenever it determines the fee is in the fund’s best interest, regardless of specific liquidity thresholds.4U.S. Securities and Exchange Commission. SEC Adopts Money Market Fund Reforms and Amendments to Form PF Reporting
The idea behind both fees is that departing shareholders should bear the cost of the portfolio sales their redemptions force, instead of diluting the value of the shares held by everyone who stays. Any imposed fee is disclosed in the fund’s Statement of Additional Information for the following ten years.
Breaking the Buck and the Real Risk
A stable-NAV fund “breaks the buck” when its share price falls below $1.00. If the NAV deviates by more than half a cent from a dollar, the fund has to reprice its shares, and investors holding at that moment take a real loss.6Investor.gov. Money Market Funds – Investor Bulletin This happened most prominently in September 2008 when the Reserve Primary Fund’s NAV fell to $0.97 after Lehman Brothers’ commercial paper became worthless.
The deeper risk isn’t the price drop itself but the run it can trigger. When investors fear a fund might break the buck, they rush to redeem before the price falls further, which forces the fund to sell holdings into a weak market and accelerates the very decline everyone is trying to escape. The Federal Reserve has identified prime funds as a structural vulnerability for exactly this reason, pointing to investor runs during September 2008 and March 2020.7Federal Reserve. Investor Base and Prime Money Market Fund Behavior The 2023 reforms were an attempt to defuse that incentive by replacing gates with fees that raise the cost of running for the exit instead of locking the door.
Institutional investors in floating-NAV funds don’t experience a dramatic breaking-the-buck moment. Gains and losses appear in the share price every day, which can actually reduce panic because no one is staring at a $1.00 price wondering if today is the day it suddenly drops.
Prime Funds Compared With Government Money Market Funds
Government money market funds invest almost entirely in Treasury securities, agency debt, and repurchase agreements backed by those instruments. Prime funds, by holding corporate and bank debt instead, pick up some credit risk in exchange for what has historically been a yield premium.
That premium is not guaranteed. SEC data showed both prime and government funds producing an asset-weighted seven-day net yield of 3.73% as of February 2026, meaning no meaningful spread existed at that point.8U.S. Securities and Exchange Commission. Money Market Funds – Yields The gap tends to widen during credit stress, when investors demand more compensation for holding corporate paper. In calm markets the extra yield can be slim.
Government funds are also exempt from the mandatory liquidity fee framework and the floating NAV requirement, regardless of share class. If you want simpler pricing and no exposure to liquidity fees, a government fund is the straightforward choice. If you’ll accept more complexity for a historically modest yield advantage, a prime fund may make sense. Taxes often tip the comparison, as discussed next.
How Prime Fund Income Is Taxed
Distributions from a prime money market fund are taxed as ordinary income at your federal marginal rate.9Internal Revenue Service. Topic No. 404, Dividends and Other Corporate Distributions Because the portfolio is corporate debt and bank instruments, nearly all the income passed through to you is interest reported as ordinary dividends. A money market fund could in theory receive some qualified dividend income taxed at lower capital gains rates, but in practice the instruments prime funds hold almost never generate qualified dividends. Expect virtually all of your distributions to be taxed at ordinary rates.
State and local taxes often matter more than the federal rate here. A government fund holding Treasuries can pass through a state tax exemption on the Treasury portion, since most states exempt interest on U.S. government obligations. Prime funds hold primarily private-sector debt, so their distributions generally receive no state tax break. In a high-tax state, if the prime-versus-government yield difference is slim, the after-tax math can actually favor the government fund.
You’ll receive a Form 1099-DIV each year reporting your ordinary dividend income and any capital gain distributions.9Internal Revenue Service. Topic No. 404, Dividends and Other Corporate Distributions
Tax Reporting for Floating-NAV Shares
If you hold an institutional prime fund with a floating NAV, each redemption can produce a small capital gain or loss against your cost basis. Without special relief that would be a recordkeeping nightmare, since shares often move daily through sweep accounts.
The IRS addressed this in Revenue Procedure 2014-45, which provides two forms of relief. First, it established a simplified method (the “NAV method”) that lets floating-NAV shareholders aggregate gains and losses over each computation period rather than tracking every individual transaction.10Internal Revenue Service. Revenue Procedure 2014-45 You calculate a net gain or loss for the period as a whole, which cuts the line items on your return dramatically.
Second, the IRS will not treat any redemption of floating-NAV money market fund shares as a wash sale under Section 1091.10Internal Revenue Service. Revenue Procedure 2014-45 Without that blanket exemption, almost every redemption followed by a repurchase within 30 days would technically trigger a wash sale, disallowing tiny losses and forcing awkward basis adjustments. The exemption means you can redeem and reinvest freely without wash sale complications.
Realized gains and losses on institutional shares are reported on Form 8949 and Schedule D.9Internal Revenue Service. Topic No. 404, Dividends and Other Corporate Distributions Most fund companies supply the needed data on year-end statements, but understanding the NAV method matters if you’re reconciling your own records or using tax software that doesn’t apply it automatically.