Prime Money Market Fund

A prime money market fund is a money market fund whose portfolio includes private-sector short-term debt, commercial paper, negotiable certificates of deposit, short-term corporate notes, and repurchase agreements, alongside any government securities it holds. That is what separates it from a government money market fund, which under SEC rules invests almost entirely in cash, government securities, and fully collateralized repurchase agreements. The older name for it, "general-purpose" fund, captures the idea: it is not restricted to a single category of issuer.

Everything a money market fund is, that it is a security rather than an insured deposit, that it is governed by SEC Rule 2a-7, and how the stable-versus- floating share price and the liquidity fees work, is covered in full on the money market fund page. This page is about the prime category specifically: what it holds, why that changes the yield and the risk, why the word "prime" does not appear in the rule, and why the 2008 reforms centered on it.

The defining choice is credit, and the reward is a modest extra yield. A government money market fund lends, in effect, almost entirely to the U.S. Treasury and government agencies. A prime fund also lends to banks and companies for very short periods. That private-sector credit is why a prime fund typically pays a little more than a government fund holding the same maturities, and why it carries a little more credit and liquidity risk in return. The gap is usually small, and it can widen or reverse when short-term credit markets are stressed, which is precisely when the difference matters most.

"Prime" is a market word, not a legal category, and this trips people up. SEC Rule 2a-7 defines only two kinds of money market fund by name, a government fund and a retail fund, and describes every other fund only negatively, as one that is neither. The market calls that residual "prime" (or "municipal," when its debt is tax-exempt), but those labels are not in the rule. This matters because the widely repeated summary that "prime funds have a floating share price" is wrong for exactly the prime funds an individual is most likely to hold. A retail prime fund, defined by being limited to individual investors, keeps a stable $1.00 price. It is the institutional prime funds, open to corporate and other non-individual investors, that must price their shares to four decimal places and can be subject to a mandatory fee on heavy redemption days. The full mechanics of both belong to the money market fund page.

The 2008 crisis is why this category is regulated the way it is. In September 2008 a large prime fund, holding short-term Lehman Brothers debt that suddenly became nearly worthless, saw its share price fall below $1.00, an event called "breaking the buck." The run that followed helped freeze the short-term funding markets that companies rely on. The SEC's later reforms, including four-decimal pricing for institutional prime and municipal funds and the liquidity fees that replaced redemption gates, were aimed at this category, because a prime fund's private-sector holdings are what can lose value in a credit panic. A government fund faces far less of that particular risk, which is why the two categories are treated differently.

You buy shares of the fund through a brokerage or fund company. The fund invests in short-term instruments within the limits Rule 2a-7 sets, including the private-sector debt that makes it a prime fund, and distributes the income it earns. If it is a retail prime fund, its price is normally reported as $1.00 and your balance is simply the number of shares; if it is an institutional prime fund, its price is computed to four decimals and moves slightly.

A hypothetical example of the yield trade-off. A government money market fund is paying an annualized 4.00% and a comparable prime fund is paying 4.25%. On a $50,000 balance held for a year, the prime fund produces about $2,125 of income against the government fund's $2,000, a difference of about $125. That $125 is the compensation for holding corporate and bank debt instead of government paper, and it is exactly the spread that can vanish, or turn painful, if that private credit comes under stress. Whether the extra yield is worth the extra risk depends on what the cash is for.

Pros

• Usually yields somewhat more than a government money market fund of similar maturity, because it holds private-sector debt.
• A retail prime fund, the version an individual typically buys, still aims to hold a stable $1.00 share price.
• Highly liquid, with shares generally redeemable the same or the next business day, which suits cash waiting for a purpose.

Cons

• It is not FDIC-insured and can lose value; its private-sector holdings are what broke the buck in 2008.
• Its yield advantage over a government fund is small and can reverse in a credit panic, exactly when safety matters most.
• An institutional prime fund prices to four decimals and can impose a mandatory fee on heavy-redemption days, so its value visibly moves.