WASHINGTON — Interest rates on short-term Treasury bills were mixed in Monday’s auction with rates on three-month bills declining to their lowest level since mid-December while six-month bills rose to their highest level in more than nine years.

The Treasury Department auctioned $48 billion in three-month bills at a discount rate of 1.425 percent, down from 1.430 percent last week. Another $42 billion in six-month bills was auctioned at a discount rate of 1.625 percent, up from 1.615 percent last week.

The three-month rate was the lowest since those bills averaged 1.355 percent on Dec. 18. The six-month rate was the highest since those bills averaged 1.800 percent on Oct. 20, 2008.

The discount rates reflect that the bills sell for less than face value. For a $10,000 bill, the three-month price was $9,963.98 while a six-month bill sold for $9,917.85. That would equal an annualized rate of 1.450 percent for the three-month bills and 1.661 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, rose to 1.80 percent last Thursday, up slightly from 1.79 percent on Jan. 19.