WASHINGTON — Interest rates on short-term Treasury bills rose in Monday’s auction with rates on six-month bills climbing to their highest level in nine years.

The Treasury Department auctioned $42 billion in three-month bills at a discount rate of 1.105 percent, up from 1.090 percent last week. Another $36 billion in six-month bills was auctioned at a discount rate of 1.245 percent, up from 1.240 percent last week.

The three-month rate was the highest since those bills averaged 1.180 percent on July 24. The six-month rate was the highest since those bills averaged 1.400 percent on Oct. 27, 2008.

The discount rates reflect that the bills sell for less than face value. For a $10,000 bill, the three-month price was $9,972.07 while a six-month bill sold for $9,937.06. That would equal an annualized rate of 1.123 percent for the three-month bills and 1.270 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 1.43 percent on Friday, little changed from 1.42 percent at the beginning of last week on Oct. 16.