NEW YORK (CNN/Money) - Long-term mortgage rates hit their lowest levels in more than six months due to slow economic growth and little worry of inflation, economists said Thursday.

Freddie Mac reported that the 30-year mortgage averaged 6.76 percent in the week ending May 31, with an average of 0.8 point payable up front to the lender. The rate slid from last week's average of 6.81 percent. The 30-year mortgage averaged 7.24 percent a year ago and has not been lower since Nov. 23, 2001, when it averaged 6.75 percent.

The 15-year fixed-rate mortgage averaged 6.22 percent, down from 6.28 percent last week and down from 6.78 percent a year ago. The 15-year averaged 0.8 point payable up front. The last time the rate was lower was on Nov. 16, 2001, when it averaged 5.98 percent.

In addition, one-year adjustable-rate mortgages indexed to the Treasury dipped to 4.76 percent, with an average 0.7 of a point due up front, down slightly from 4.85 percent last week and down from an average of 5.89 percent a year ago.

"Slower economic growth this quarter and little or no inflation worries allowed rates to drift downward these last few weeks to the benefit of homebuyers," Freddie Mac chief economist Frank Nothaft said. "As a matter of fact, low mortgage rates induced an unexpectedly high level of new and existing home sales last month." (Read more about increasing the retail value of a home.)

Nothaft added that more homebuyers are opting for the one-year adjustable-rate mortgage, with this type of mortgage rising to 20 percent of the market in April from 10 percent in the same month last year.

Calculators: Your Home       Rent vs.Buy Mortgage Qualifier Refinancing calculator Rate Search Tool

Freddie Mac (FRE: down $0.06 to $64.94, Research, Estimates), or Federal Home Mortgage Corp., is a publicly traded company the government established in 1970 to provide a flow of funds to mortgage lenders. It buys mortgages from banks, bundles them and then resells them as mortgage-backed securities.

Its products, and the products of other similar entities, have become increasingly popular as an alternative to government-backed bonds, particularly with international investors.