The lower level of net interest income in 2010 when compared to 2009 was the result of an 11 basis point narrowing of the net interest margin in 2010 to 4.04%. The lower margin resulted principally from a reduction in the securities portfolio average yield during the fourth quarter of 2010 versus 2009. Somewhat offsetting the lower securities portfolio yield was a reduction in the average cost of deposits, our primary funding source, in 2010.
The Company's fourth quarter 2010 average interest-earning asset yield was 4.75% versus 5.03% in the comparable 2009 period. The average yield on loans increased by six basis points in the fourth quarter of 2010 to 5.48%. This improvement was offset by a 112 basis point decline in the average yield on the Company's securities portfolio to 3.02% in the fourth quarter of 2010 versus 2009. The securities portfolio (excluding Federal Home Loan Bank and other restricted stock), has an unrealized positive pre-tax mark to fair value of $5 million at December 31, 2010 and an estimated weighted average life of 3.1 years, decreased by $33 million to $383 million at December 31, 2010 versus the comparable 2009 date and also decreased by $43 million from September 30, 2010.
The Company's average cost of interest-bearing liabilities declined 26 basis points to 0.99% in the fourth quarter of 2010 versus 1.25% in the fourth quarter of 2009. The lower cost of funds resulted from growth in average savings account balances in 2010 coupled with the Company's ongoing management of deposit rates during the past year as deposit pricing continues to ease in local markets. Total deposits were unchanged at $1.3 billion at December 31, 2010 versus December 31, 2009 but declined by $37 million when compared to September 30, 2010. Also contributing to the reduction in 2010 funding costs was the Company's December 2009 exchange of its high-cost $10 million, 8.25% subordinated notes for newly issued common stock. The subordinated notes were due to mature in 2013.