Money Market Fund
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 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 ("stable NAV"). This makes them feel and function like a bank account while typically paying significantly more interest.
What Money Market Funds Hold
| Security Type | Description | Typical % |
|---|---|---|
| US Treasury bills | Government debt maturing in weeks to months | 20-100% (for government MMFs) |
| Government agency securities | Fannie Mae, Freddie Mac short-term notes | 0-30% |
| Repurchase agreements (repos) | Overnight lending backed by Treasuries | 20-60% |
| Commercial paper | Corporate short-term unsecured debt | 0-40% (prime MMFs only) |
| Certificates of deposit | Bank CDs (short-term) | 0-20% (prime MMFs only) |
| Municipal notes | Short-term government notes | 0-100% (muni MMFs) |
Types of Money Market Funds
| Type | Holdings | Tax Treatment | Best For |
|---|---|---|---|
| Government MMF | Treasuries + agency debt + repos | Interest taxable federally; state-exempt | Most investors; safest |
| Prime (General Purpose) MMF | Adds commercial paper + CDs | Fully taxable | Slightly higher yield seekers |
| Tax-Exempt Municipal MMF | Municipal securities | Federal tax-exempt | High-income investors in high tax brackets |
| Treasury-Only MMF | Only US Treasuries | Federal taxable; state-exempt | Maximum safety + state tax savings |
Money Market Funds vs. Money Market Accounts
Despite similar names, these are completely different products:
| Feature | Money Market Fund (MMF) | Money Market Account (MMA) |
|---|---|---|
| Offered by | Brokerage / mutual fund company | Bank or credit union |
| FDIC/NCUA insured | No (not insured) | Yes (up to $250,000) |
| Regulated by | SEC (under Investment Company Act) | FDIC/OCC |
| Principal protection | Stable $1 NAV (not guaranteed) | Guaranteed |
| "Breaking the buck" risk | Yes (rare but possible) | No |
| Typical yield (2024) | 4.75-5.25% | 4.25-5.00% |
| Check writing | Often available | Often available |
| Required minimum | $0-$3,000 | $0-$2,500 |
Yields and Comparative Rates (2024)
| Product | Approximate Yield |
|---|---|
| Big bank savings account | 0.01-0.06% |
| High-yield savings account | 4.50-5.25% |
| Government money market fund (Fidelity SPAXX) | 4.95-5.10% |
| Treasury-only MMF (Fidelity FDLXX) | 4.85-5.00% |
| Prime MMF | 5.00-5.20% |
| 3-month Treasury bill | 5.15-5.35% |
Major Money Market Funds (2024)
| Fund | Ticker | Type | AUM | Yield |
|---|---|---|---|---|
| Fidelity Government Money Market | SPAXX | Government | $275B+ | ~5.0% |
| Fidelity Treasury Money Market | FDLXX | Treasury-only | $65B+ | ~4.9% |
| Vanguard Federal Money Market | VMFXX | Government | $250B+ | ~5.1% |
| Schwab Value Advantage Money Fund | SWVXX | Prime | $165B+ | ~5.15% |
| Vanguard Treasury Money Market | VUSXX | Treasury-only | $75B+ | ~4.9% |
"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" — 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 $310B in institutional MMF withdrawals in 3 days, necessitating a Treasury guarantee program
Post-2008 reforms: SEC Rule 2a-7 imposed significant reforms: floating NAV for prime institutional funds, gates and redemption fees during stress, stricter liquidity requirements. Government MMFs (most retail funds) still maintain the stable $1 NAV.
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:
| State | State Tax Rate | After-Tax Yield Advantage |
|---|---|---|
| California | 13.3% | Treasury MMF yield effectively 15%+ higher than HYSA |
| New York | 10.9% | Meaningful advantage |
| Texas/Florida | 0% | No advantage |
Example: 5.0% Treasury MMF yield for a California investor at 37% federal + 13.3% state:
- HYSA: 5.0% × (1 - 0.37 - 0.133) = 2.49% after-tax
- Treasury MMF: 5.0% × (1 - 0.37) = 3.15% after-tax (no state tax)
- After-tax advantage: 0.66% annually just from state tax exemption
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 comprehensive SEC reforms
- Yields track the federal funds rate closely — they rise when the Fed hikes and fall when the Fed cuts
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 higher 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 (or rise), money market funds win. If you want to lock in today's rates before the Fed cuts, CDs may be preferable.
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 1-2 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.
Related Terms
Money Market Account
A money market account is an FDIC-insured bank deposit account that combines features of savings and checking accounts — offering higher interest rates than standard savings accounts with limited check-writing and debit card access.
Mutual Fund
A mutual fund pools money from many investors to buy a diversified portfolio of stocks, bonds, or other securities, managed by professional portfolio managers.
Balanced Fund
A balanced fund is a mutual fund that holds a mix of stocks and bonds in a fixed or target ratio — typically 60% equities and 40% fixed income — providing both growth potential and income in a single, diversified investment.
Acid-Test Ratio
The acid-test ratio measures a company's ability to meet short-term obligations using only its most liquid assets — cash, short-term investments, and receivables — excluding inventory that may not be quickly converted to cash.
Liquidity
Liquidity refers to how quickly and easily an asset can be converted to cash without significantly affecting its price, determining how accessible your money is when you need it.
Expense Ratio
An expense ratio is the annual fee charged by a mutual fund or ETF as a percentage of your investment, covering management, administration, and operational costs — and it compounds quietly into massive wealth differences over decades.
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