WASHINGTON(AP)—Interest rates on short-term Treasury bills fell in Monday’s auction. The rate on six-month bills dropped to the lowest level since late December, while three-month bills also dipped.

The Treasury Department auctioned $31 billion in six-month bills at a discount rate of 0.285 percent, down from 0.350 percent last week. Another $32 billion in three-month bills was auctioned at a discount rate of 0.190 percent, down from 0.195 percent last week.

The six-month rate was at its lowest level since it averaged 0.250 on Dec. 29. The three-month rate was at its lowest level since those bills averaged 0.160 on June 15.

The discount rates reflect that the bills sell for less than face value. For a $10,000 bill, the three-month price was $9,995.20, while a six-month bill sold for $9,985.59. That would equal an annualized rate of 0.193 percent for the three-month bills, and 0.289 percent for the six-month bills.

Separately, the Fed said Monday that the average yield for one-year Treasury bills, a popular index for making changes in adjustable rate mortgages, rose to 0.51 percent last week from 0.44 percent the previous week.