Yields fell sharply at the Treasury's latest weekly sale of short-term bills yesterday.

The average return on the 13-week bills fell to 6.5 per cent from 8.035 per cent at last week's auction and was the lowest since 6.476 per cent on May 22, 1978.

The average return on the 26-week bill dropped to 6.935 per cent from 8.165 per cent a week earlier and was the lowest since 6.777 per cent on April 24, 1978.

For the 13-week bills, the Treasury received $6.82 billion in tenders and accepted $2.8 billion.

Meanwhile, the Treasury said that the average 2 1/2-year Treasury yield curve rate during the five days ended Monday was 9 percent. The rate is used by federal regulators to determine maximum interest payable on savings certificates issued during the next two weeks in denominations of less than $100,000 and maturities of 2 1/2 years or more.

Under new rules, the commercial bank ceiling for the certificates is one-quarter percentage point below the yield, and the ceiling for thrift institutions is the yield.

However, under new rules, commercial banks may pay 9.25 percent and thrifts 9.5 pecent, even when the Treasury yield falls below 9.5 percent.

And the Treasury also said that the acceptance of mail orders for Marion Anderson half-ounce and Grant Wood one-ounce gold medallions has been postponed to July 15 from next Monday due to delays in printing and distributing order blanks to Post Offices.