WASHINGTON — Rates on 30-year mortgages fell this week to the lowest level in three months.

Freddie Mac, the mortgage company, reported yesterday that 30-year fixed-rate mortgages averaged 6.45 percent. That was down from 6.52 percent last week and was the lowest level since the week of May 31, when rates stood at 6.42 percent.

The moderation provides welcome news for prospective homebuyers, many of whom are finding it harder to obtain loans as lenders tighten up on borrowing standards in the face of rising loan delinquencies.

Rates in other mortgage categories were mixed last week.

Rates on 15-year fixed-rate mortgages, a popular choice for refinancing, averaged 6.12 percent, down from 6.18 percent last week.

But rates on five-year adjustable-rate mortgages rose slightly to 6.35 percent, compared with 6.34 percent last week.

Rates rose more steeply for one-year adjustable-rate mortgages, climbing to 5.84 percent, up from 5.60 percent last week.

The big drop in 30-year mortgage rates followed the Aug. 17 decision by the Federal Reserve to slice its discount rate, the interest it charges to make direct loans to banks. That move was designed to calm recent turmoil on Wall Street about a spreading credit crunch.

Many economists believe the Fed will soon decide to cut its more economically significant federal funds rate fund, a key benchmark for millions of consumer and business loans.

The mortgage rates do not include add-on fees known as points. Thirty-year mortgages and 15-year mortgages each carried a nationwide average fee of 0.5 point. Five-year ARMs had an average fee of 0.6 point, while one-year ARMs carried an average fee of 0.8 point.

A year ago, rates on 30-year mortgages stood at 6.44 percent, 15-year mortgages were at 6.14 percent, five-year ARMS averaged 6.11 percent and one-year ARMs were at 5.59 percent.

After a five-year boom, sales of both new and existing homes fell sharply last year. The slump has gotten worse this year as lenders have tightened standards amid soaring foreclosures and late payments.

Those problems began in the market for subprime loans, which are offered to borrowers with weak credit histories, but have now spread to other loan categories.