Treasury-Only Money Market Fund
A Treasury-only money market fund is a money market mutual fund that invests exclusively in direct obligations of the US Treasury — bills, notes, and bonds with very short remaining maturities — providing maximum credit safety, federal-government backing, and interest income exempt from state and local income taxes.
Money market funds come in multiple varieties — government funds, prime funds, and Treasury-only funds — with important differences in credit risk, tax treatment, and regulatory classification. Treasury-only funds occupy the most conservative end of the spectrum, holding only securities with the direct backing of the US federal government. They exclude agency securities (like Fannie Mae or Freddie Mac debt), repurchase agreements, and any corporate paper.
The state and local tax exemption is a meaningful advantage for residents of high-tax states. Interest from US Treasury obligations is exempt from state and local income tax under federal law. A Treasury-only money market fund passes this exemption through to shareholders, while a government money market fund that holds agency bonds or repos does not necessarily qualify for the same exemption on those portions of the portfolio. Investors in high-tax states should verify what percentage of a fund's income qualifies for state tax exemption — fund companies typically publish this figure annually for tax reporting purposes.
Safety is the defining characteristic. Treasury-only funds have never broken the dollar (experienced net asset value below $1.00 per share) in their history. The 2008 financial crisis saw the Reserve Primary Fund break the dollar due to Lehman Brothers commercial paper exposure — an event impossible in a Treasury-only fund. During severe market stress, Treasury-only money market funds often see inflows as investors seek the safest liquid harbor, while prime funds (which hold commercial paper) can face redemption pressure.
The tradeoff is yield. Because Treasury-only funds exclude higher-yielding agency and corporate instruments, their yields typically run somewhat below prime and government money market funds when credit spreads are normal. In risk-off environments, the yield gap often narrows or reverses as demand drives Treasury yields down and credit spreads widen.
For investors using money market funds as an emergency fund, cash management account, or brokerage sweep, selecting a Treasury-only fund aligns the highest safety requirement with the purpose of the holding. Taxable investors in states like California, New York, or Minnesota capture additional after-tax value from the state tax exemption.