Treasury bills fell at the government's weekly auction yesterday, for the second time in the past four weeks.

The government sold $4.3 billion of 3-month bills at an average discount of 15.235 percent, down from 15.674 percent a week earlier.

The government also sold $4.3 billion worth of 6-month bills at an average discount of 15.122 percent, down from 15.571 percent a week ago.

For the 26-week bills, the Treasury recieved $9.16 billion in tenders and accepted $4.3 billion at returns ranging from 15.098 percent to 15.138 percent, including $901.2 million of non-competitive tenders at the average return. The Treasury accepted 58 percent of the tenders at the highest return. The average price was 92.355 percent.

The rates are the lowest since July 27, when 13-week bills yeilded 15.065 percent and 26-week bills yeilded 14.790 percent.

The new 6-month T-bill rate, plus a guarter of a percentage point, is 15.372 percent. The current interest ceiling for these certificates is 15.821 percent.

The ceiling rate for 2 1/2-year "Small Savers" certificates, available denominations as small as $500, was freed from its former lower levels on Aug. 1 in a government deregulatory action. The rate, now pegged to that for comparable Treasury securities, is 15.80 percent at thrift institutions and 15.55 at commercial banks.

The latest Treasury bill auction will raise 450 million in new cash for the Treasury after maturing bills are paid off.