WASHINGTON — Interest rates on short-term Treasury bills were mixed in Monday's auction, with rates on six-month bills rising while rates on three-month bills were unchanged from last week.

The Treasury Department auctioned $48 billion in three-month bills at a discount rate of 1.910 percent, the same as last week. Another $42 billion in six-month bills was auctioned at a discount rate of 2.075 percent, up from 2.070 percent last week.

The six-month rate was the highest since those bills averaged 2.080 percent three weeks ago on May 21.

The discount rates reflect that the bills sell for less than face value. For a $10,000 bill, the three-month price was $9,951.72 while a six-month bill sold for $9,895.10. That would equal an annualized rate of 1.946 percent for the three-month bills and 2.126 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, stood at 2.30 percent on Friday, unchanged from the beginning of last week on June 4, when the yield was also 2.30 percent.