Money Market Fund

Quick Definition

A money market fund (MMF) is a type of mutual fund that invests exclusively in short-term, high-quality debt instruments, such as Treasury bills, commercial paper, and repurchase agreements, with the goal of maintaining a stable net asset value (NAV) of $1.00 per share while paying competitive short-term interest rates.

What It Means

Money market funds occupy the intersection between investing and banking. They are SEC-regulated investment products (not FDIC-insured bank accounts) that function like a high-yield savings account. You put money in, earn competitive interest, and can withdraw at any time. They are the standard cash management vehicle at brokerage firms like Fidelity, Vanguard, and Schwab.

The key feature: unlike bond funds or stock funds whose share prices fluctuate, money market funds are designed to always be worth $1.00 per share. This makes them feel and function like a bank account while typically paying competitive yields.

In July 2026, total money market fund assets stand at $7.86 trillion, according to the Investment Company Institute. Government funds hold $6.48 trillion, prime funds hold $1.23 trillion, and tax-exempt funds hold $148.6 billion. Yields track the federal funds rate closely, so with the Fed holding at 3.50-3.75%, MMF yields range from approximately 3.5% to 4.0% depending on fund type and expenses.

What Money Market Funds Hold

Types of Money Market Funds

Money Market Funds vs. Money Market Accounts

Despite similar names, these are completely different products:

Yields and Comparative Rates (July 2026)

Yields have declined from their 2023-2024 peaks above 5% as the Fed cut rates from 5.50% to 3.50-3.75%. If the Fed holds or cuts further in 2027, MMF yields will stay at or below current levels.

"Breaking the Buck": The Key Risk

Money market funds aim to maintain $1.00 NAV, but this is not guaranteed. If fund holdings decline in value, the NAV can "break the buck" and fall below $1.00.

Historical breaks:

• 1994: Community Bankers U.S. Government Money Market Fund (minor; institutional only)
• 2008: Reserve Primary Fund broke below $0.97 after holding Lehman Brothers commercial paper. The panic triggered $310 billion in institutional MMF withdrawals in 3 days, necessitating a Treasury guarantee program.

SEC Reforms: 2010, 2014, and 2023

Money market funds have been reformed three times since the 2008 crisis:

The 2023 reforms are the most recent. The SEC found that the 2014 gate and fee provisions actually amplified investor runs in March 2020, as investors rushed to redeem before gates could be imposed. The new reforms remove gates entirely and replace them with dynamic liquidity fees that charge redeeming investors for the cost of providing liquidity during stress periods. Government MMFs (most retail funds) still maintain the stable $1 NAV and are exempt from the mandatory fee requirement.

State Tax Advantage of Government/Treasury MMFs

Interest from US Treasury obligations is exempt from state and local income taxes. For investors in high-tax states, this creates a meaningful after-tax advantage:

Example: 3.80% Treasury MMF yield for a California investor at 37% federal + 13.3% state:

• HYSA: 3.80% x (1 - 0.37 - 0.133) = 1.89% after-tax
• Treasury MMF: 3.80% x (1 - 0.37) = 2.39% after-tax (no state tax)
• After-tax advantage: 0.50% annually just from state tax exemption

On a $50,000 balance, that is $250 per year in tax savings on top of the competitive yield.

Key Points to Remember

• Money market funds invest in short-term, high-quality debt to maintain a stable $1/share NAV
• They are not FDIC-insured, unlike bank money market accounts
• Government MMFs (Treasuries + agency debt) are the safest; prime MMFs add commercial paper for slightly higher yield
• Treasury-only MMFs offer state tax exemption on interest, valuable in high-tax states
• The 2008 Reserve Primary Fund "breaking the buck" led to three rounds of SEC reforms, most recently in 2023
• Yields track the federal funds rate closely: they rise when the Fed hikes and fall when the Fed cuts
• Total MMF assets reached $7.86 trillion in July 2026, per ICI data

Common Mistakes to Avoid

• Assuming your MMF is FDIC insured: It is not. Money market funds are SEC-regulated investment products. If you want FDIC insurance, use a bank money market account or savings account instead. The risk of loss is small but real, as the 2008 Reserve Primary Fund demonstrated.
• Ignoring the expense ratio: MMFs charge an expense ratio that reduces your yield. A fund earning 3.80% gross with a 0.40% expense ratio nets you 3.40%. Compare expense ratios across similar fund types. Vanguard and Fidelity typically offer the lowest expenses in the industry.
• Holding too much in MMFs when you have a long time horizon: In July 2026, MMFs yield around 3.50-4.00%. Over 10+ years, a diversified stock portfolio has historically returned 8-10% annually. Keeping your entire portfolio in cash equivalents because they feel safe means sacrificing significant long-term growth. Use the compound interest calculator to see the difference.
• Forgetting about transfer delays: Moving money from a brokerage MMF to your bank account takes 1 to 2 business days. If you need cash instantly for an emergency, keep at least one month of expenses in a bank account with same-day access.

Frequently Asked Questions

Q: Is a money market fund the same as a savings account? A: No. A money market fund is a mutual fund, not FDIC-insured, not a bank product. A savings account is a bank deposit covered by FDIC insurance up to $250,000. Money market funds typically pay competitive yields and offer similar liquidity, but without the FDIC guarantee. For emergency funds, many people split between FDIC-insured HYSA and brokerage money market funds.

Q: When should I use a money market fund vs. a CD? A: Money market funds provide immediate liquidity with yields that float with rates. CDs lock in a fixed rate for a set term but incur penalties for early withdrawal. If you need flexibility and expect rates to stay high, money market funds win. If you want to lock in today's rates before the Fed cuts, CDs may be preferable. See our CD ladder strategy guide for more.

Q: Are money market funds good for an emergency fund? A: They are reasonable for the portion of your emergency fund held at a brokerage. The limitations: they are not FDIC-insured, and fund transfers to a bank account take 1 to 2 business days. For immediate liquidity in an emergency, keep at least 1 month of expenses in a bank HYSA. Use money market funds at your brokerage for the rest. Use the emergency fund calculator to determine your target.

Q: What happened to money market funds in 2020? A: In March 2020, institutional prime money market funds experienced large outflows as investors fled to cash during the COVID panic. The 2014 SEC reforms that linked redemption gates to liquidity thresholds actually made the run worse, as investors rushed to redeem before gates could be imposed. The Federal Reserve had to establish a liquidity facility for MMFs. This led to the 2023 SEC reforms that removed gates and replaced them with dynamic liquidity fees.