WASHINGTON — Interest rates on short-term Treasury bills were mixed in Monday’s auction with rates on three-month bills declining while rates on six-month bills were unchanged.

The Treasury Department auctioned $39 billion in three-month bills at a discount rate of 1.000 percent, down from 1.015 percent last week. Another $33 billion in six-month bills was auctioned at a discount rate of 1.115 percent, unchanged from last week.

The three-month rate was the lowest-highest since those bills averaged 0.990 percent 10 weeks ago on June 12.

The discount rates reflect that the bills sell for less than face value. For a $10,000 bill, the three-month price was $9,974.44 while a six-month bill sold for $9,943.63. That would equal an annualized rate of 1.016 percent for the three-month bills and 1.137 percent for the six-month bills.

Separately, the Federal Reserve said Monday that the average yield for one-year Treasury bills, a popular index for making changes in adjustable rate mortgages, edged up to 1.24 percent last Thursday, up slightly from 1.21 percent on Aug. 11.