Treasury Bill vs Money Market Fund

A treasury bill vs money market fund comparison matters when parking short-term cash: T-bills are direct government debt maturing in one year or less; money market funds are mutual funds holding baskets of short-term instruments. Both aim for safety and liquidity, but differ in price discovery, tax treatment, and the exact risks you bear.

What each vehicle does

Treasury bills are short-term IOUs from the U.S. government, issued directly at auction every week. You pay less than face value (e.g., pay $99.50 for a $100 bill), and at maturity, the Treasury pays you the full $100. The difference is your return, called the “discount.” Maturities come in four standard tenors: 4, 13, 26, and 52 weeks. Bills are held in TreasuryDirect (free, government account) or in a brokerage.

Money market funds are mutual funds or ETFs that hold portfolios of short-term, high-quality debt. Their fund manager buys Treasury bills, commercial paper (corporate IOUs), repurchase agreements (short-term loans secured by Treasury collateral), and certificates of deposit from banks. Each day, the fund’s Net Asset Value (NAV) resets to reflect the current value of all holdings. You buy and sell shares at that NAV.

Yield and return

T-bills offer a single, locked-in yield known at purchase. You buy at an auction yield; that’s your return unless you sell early in the secondary market. The yield moves daily—it depends on what new auctions are pricing at—but once you own a bill, the math is certain.

Money market funds show a “current yield” that floats daily and often misleads. What matters is the “yield to maturity” (average return if you hold to the fund’s weighted maturity), but even that is a forecast because the manager constantly rolls holdings and rates shift. The fund’s actual return depends on which bonds the manager holds, their credit risk, and fee drag.

For example, if a 13-week T-bill is auctioning at a 4.5% discount rate and a prime money market fund is showing 4.2% yield, the T-bill locks in higher return. But if rates fall during your holding period, a T-bill’s value rises if you sell it early, and the fund NAV would rise as well—the T-bill wins on price appreciation, though both instruments are stable.

Credit risk and safety

Treasury bills carry zero default risk; the U.S. government backs them. That’s not true for money market funds. A prime MMF holds commercial paper (unsecured corporate debt) and repo. In a credit stress—say a major bank fails or a large corporation implodes—the fund faces mark-to-market losses. The 2008 financial crisis and the 2023 bank failures showed this: money market funds experience outflows when credit risk rises, and some have imposed redemption restrictions.

Government-only money market funds exist (they hold T-bills and repo backed by Treasuries) and eliminate credit risk almost entirely, but their yields are slightly lower because they exclude the (small) premium from holding corporate paper.

Liquidity and daily access

T-bills traded at auction are illiquid until maturity—you own them until the date printed on the certificate. If you need cash before that date, you must sell in the secondary market, which could be at a small loss if rates have moved against you. The secondary market is liquid (traders buy and sell T-bills constantly), but bid-ask spreads can widen in market stress.

Money market funds are designed for daily redemption. The SEC requires them to maintain a short weighted average maturity (typically under 60 days for prime funds, even shorter for government funds) so the fund can raise cash on demand without selling illiquid assets. You can typically redeem shares same-day or next business day. This flexibility comes at a cost: the fund cannot lock in returns like a T-bill does.

Tax treatment

Interest from T-bills is subject to federal income tax but exempt from state and local taxes. This is a significant advantage for residents of high-tax states like New York, California, and New Jersey. On a 4% T-bill yield, state tax exemption might be worth 0.5–1.0% extra return annually depending on your bracket and state rate.

Interest from money market funds is fully taxable at federal, state, and local rates. Some MMFs hold municipal securities and offer tax-exempt yields, but those are smaller and usually offer lower absolute returns.

Market mechanics: auctions vs. continuous pricing

T-bills trade at Treasury auctions held every week. Institutional investors and individuals submit competitive or non-competitive bids. Once issued, secondary-market trading is continuous over-the-counter (OTC); brokers quote bid-ask prices. You see the price before you execute.

Money market funds price once daily after market close. You place a redemption request during trading hours, and you receive the day’s NAV price—not a bid-ask spread, but a single, published price. This removes guesswork but also locks you in; you cannot time intraday price movements.

Choosing between them

If you want certainty of return and can hold cash for a defined short period, T-bills are simpler and often offer better yields. They’re ideal if you know you’ll need the money in 13 weeks or 26 weeks and want to lock in today’s rate.

If you value daily access (even at a small yield cost), need flexibility because the exact redemption date is uncertain, or want to avoid reinvestment decisions as one instrument matures, a money market fund is more practical. Prime MMFs also offer higher yields than ultra-short bond funds because they hold riskier (but still short-term) instruments like commercial paper.

For maximum safety with daily liquidity, a government-only money market fund splits the difference: it holds T-bills and Treasury repo, has near-zero credit risk, redeems daily, but forfeits the state-tax exemption advantage of direct T-bill ownership.

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