WASHINGTON — Interest rates on short-term Treasury bills were mixed in Monday’s auction, with rates on three-month bills falling to their lowest level in two weeks while rates on six-month bills were unchanged.

The Treasury Department auctioned $39 billion in three-month bills at a discount rate of 1.070 percent, down from 1.180 percent last week. Another $33 billion in six-month bills was auctioned at a discount rate of 1.130 percent, unchanged from last week.

The three-month rate was the lowest since three-month bills averaged 1.050 percent two weeks ago on July 17.

The discount rates reflect that the bills sell for less than face value. For a $10,000 bill, the three-month price was $9,972.95 while a six-month bill sold for $9,942.87. That would equal an annualized rate of 1.088 percent for the three-month bills and 1.152 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 1.22 percent last Thursday, unchanged from the level at the end of the previous week.