WASHINGTON (AP) — Interest rates on short-term Treasury bills were mixed in Monday's auction with rates on three-month bills rising to their highest level since late March. while rates on six-month bills fell.

The Treasury Department auctioned $31 billion in three-month bills at a discount rate of 0.275 percent, up from 0.240 percent last week. Another $26 billion in six-month bills was auctioned at a discount rate of 0.370 percent, down from 0.380 percent last week.

The three-month rate was the highest since those bills averaged 0.300 percent on March 28. The six-month rate was the lowest since those bills averaged 0.350 percent four weeks ago on April 18.

The discount rates reflect that the bills sell for less than face value. For a $10,000 bill, the three-month price was $9,993.05, while a six-month bill sold for $9,981.29. That would equal an annualized rate of 0.279 percent for the three-month bills and 0.376 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 0.53 percent last week from 0.52 percent the previous week.