The monthly Treasury average now stands at its lowest point in three-and-one-half years.
Last month, MTA fell to 2.47833%, according to Federal Reserve data.
During August, the index was 2.66417%, the data indicated. In September 2007, MTA was 4.8633%.
MTA hasn't been this low since March 2005, when it stood at 2.3467%.
The index is calculated based on the 12-month average of the 1-year Treasury bill's monthly average, which was 1.91% in September. The daily 1-year yield closed at 1.28% yesterday.
Another adjustable-rate mortgage index is the cost of funds index, which was reported at 2.693% in August by the Federal Home Loan Bank of San Francisco. COFI currently stands at its lowest level since June 2005, when it stood at 2.676%.
The 6-month London Interbank Offered Rate was 4.02% as of yesterday, Bankrate.com reported. LIBOR has risen each of the past three weeks.
ARM applications accounted for 2% of total applications in the Mortgage Bankers Association's Weekly Mortgage Applications Survey for the week ending Oct. 3, down from 3% the previous week.