The main differences between the types of money market funds are what each fund is allowed to invest in (i.e., the investible universe), and the corresponding regulatory rules applicable to each sector, which can affect the yield the portfolio generates. A prime money fund invests primarily in taxable short-term obligations issued by corporations and banks, as well as repurchase agreements and asset-backed commercial paper. Government and Treasury funds invest primarily in short-term U.S. government debt securities. Treasury money market funds typically limit their holdings to only U.S. Treasury obligations or repurchase agreements collateralized by U.S. Treasury securities. Municipal money market funds invest primarily in short-term, municipal money market securities issued by states, local governments, and other municipal agencies. They pay interest that is generally exempt from federal income tax. Go here for a list of current money market fund options and yields.