WASHINGTON — Interest rates on short-term Treasury bills fell in Monday’s auction, with rates on six-month bills dropping to the lowest level in four weeks.

The Treasury Department auctioned $32 billion in three-month bills at a discount rate of 0.105 percent, down from 0.125 percent last week. Another $28 billion in six-month bills was auctioned at a discount rate of 0.150 percent, down from 0.160 percent last week.