Money market mutual funds receive the reverse repurchase agreement, a.k.a. the reverse repo rate, for funds held by the Fed overnight, so it functions as a rough floor on yields.

That rate stands at 0.8 percent. It was close to zero for many months. 

At the moment, short-term Treasury bills, with yields in the 0.85 - 1.05 percent range, provide a practical ceiling for funds that hold government securities.

If the operating expenses of money market funds exceed the income they bring in, the funds could theoretically pay negative yields.

Money market mutual funds are convenient for individual investors and big institutions to keep money temporarily. However, their yields have been very low for years.

The Federal Reserve has begun to increase the short-term interest rate it controls, and money market fund yields available to...