Mortgage rates in the US fell last week, with the average rate on 30-year fixed-rate mortgages falling to 4.78 percent, a slight decline from the prior week’s 4.8 percent average and 5.06 percent in 2010, according to Freddie Mac’s weekly mortgage rates survey.

Meanwhile, rates on 15-year fixed-rate mortgages stood at 3.97 percent, slipping from 4.02 percent in the previous week and 4.39 percent in 2010.

“Mortgage rates followed Treasury bond yields lower this week amid weak local economic data reports on business conditions and house prices,” said Frank Nothaft, the Chief Economist of Freddie Mac.

Rates have slumped for months, setting record lows in the process, as yields on Treasurys fell amid economic uncertainty. But yields started to increase at the end of August. Mortgage rates generally follow the yields, which move inversely to Treasury prices.

Five-year Treasury-indexed hybrid adjustable-rate mortgages averaged 3.51 percent, down from the previous week’s 3.61 percent and four percent in 2010.

One-year Treasury-indexed ARMs were 3.15 percent, a decrease from the prior week’s 3.16 percent and 4.25 percent in 2010.

To obtain the rates, the fixed-rate mortgages required payment of an average 0.7 point, while needed an average 0.6 point. A point is one percent of the mortgage amount, charged as prepaid interest.