WASHINGTON — 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 July while rates on six-month bills dipped slightly.

The Treasury Department auctioned $42 billion in three-month bills at a discount rate of 1.050 percent, up from 1.045 percent last week. Another $36 billion in six-month bills was auctioned at a discount rate of 1.170 percent, down from 1.180 percent last week.

The three-month rate was the highest since those bills averaged 1.070 percent on July 31. The six-month rate was the lowest since those bills averaged 1.140 percent two weeks ago on Sept. 11.

The discount rates reflect that the bills sell for less than face value. For a $10,000 bill, the three-month price was $9,973.46 while a six-month bill sold for $9,940.85. That would equal an annualized rate of 1.067 percent for the three-month bills and 1.193 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 1.30 percent on Friday, unchanged from the beginning of last week on Sept. 18.