WASHINGTON — Interest rates on short-term Treasury bills fell in Monday’s auction to the lowest levels in two weeks.

The Treasury Department auctioned $51 billion in three-month bills at a discount rate of 1.760 percent, down from 1.780 percent last week. Another $45 billion in six-month bills was auctioned at a discount rate of 1.895 percent, down from 1.950 percent last week.

The three-month rate was the lowest in two weeks, since these bills averaged 1.670 percent on March 12. The six-month rate was the lowest since these bills averaged 1.850 percent, also two weeks ago on March 12.

The discount rates reflect that the bills sell for less than face value. For a $10,000 bill, the three-month price was $9,955.51 while a six-month bill sold for $9,904.20. That would equal an annualized rate of 1.792 percent for the three-month bills and 1.940 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 2.04 percent on Friday, down from 2.08 percent at the beginning of last week on March 19.