Interest expense on deposits was $4.8 million for the third quarter of 2020, compared to $6.2 million for the third quarter of 2019. The $1.4 million, or 22.1%, decrease in interest expense on deposits was primarily due to a 68 basis point decrease in the average rate paid on interest bearing deposits. The average balance of interest bearing deposits increased $291.2 million, or 22.3%, to $1.60 billion in the third quarter of 2020, compared to $1.31 billion for the third quarter of 2019. The increase in the average balance of interest bearing deposits resulted from growth among all interest bearing deposit types over the year. The average rate paid on interest bearing deposits decreased from 1.89% in the third quarter of 2019 to 1.21% in the third quarter of 2020. The decrease in the average rate paid was primarily due to the decline of market interest rates.
Interest expense on borrowings increased $546,000 to $2.0 million for the third quarter of 2020, compared to $1.4 million for the third quarter of 2019. This increase was primarily due to a higher average balance of subordinated debentures, partially offset by a decrease in FHLB advances. The higher subordinated debentures balance was due to the issuance of $50.0 million of subordinated debentures during the second quarter of 2020.
Nine Months Ended September 30, 2020 Compared to Nine Months Ended September 30, 2019
Net interest income was $63.1 million for the nine months ended September 30, 2020, an increase of $8.9 million, or 16.5%, compared to $54.2 million for the nine months ended September 30, 2019. The increase in net interest income was primarily attributable to growth in average interest earning assets due to organic growth in the loan portfolio and lower rates paid on deposits due to the falling interest rate environment. The Company anticipates that its net interest income will be adversely affected in future periods as a result of the COVID-19 pandemic and the effects of lower interest rates.
Net interest margin (on a fully tax-equivalent basis) for the nine months ended September 30, 2020 was 3.41%, compared to 3.57% for the nine months ended September 30, 2019, a decrease of 16 basis points. Despite a significant reduction in interest bearing liability costs over the year, the historically low interest rate environment, coupled with a more liquid balance sheet mix, pressured earning asset yields lower and ultimately compressed the net interest margin year-over-year.
Average interest earning assets for the nine months ended September 30, 2020 increased $443.3 million, or 21.5%, to $2.50 billion from $2.06 billion for the nine months ended September 30, 2019. This increase in average interest earning assets was primarily due to continued organic growth in the loan portfolio and the funding of PPP loans. Average interest bearing liabilities increased $280.0 million, or 19.4%, to $1.72 billion for the nine months ended September 30, 2020, from $1.44 billion for the nine months ended September 30, 2019. The increase in average interest bearing liabilities was primarily due to increases in interest bearing deposits, FHLB advances and subordinated debentures, offset partially by decreases in federal funds purchased and notes payable.
Average interest earning assets produced a tax-equivalent yield of 4.53% for the nine months ended September 30, 2020, compared to 5.01% for the nine months ended September 30, 2019. The average rate paid on interest bearing liabilities was 1.63% for the nine months ended September 30, 2020, compared to 2.06% for the nine months ended September 30, 2019.
Interest Income. Total interest income on a tax-equivalent basis was $84.8 million for the nine months ended September 30, 2020, compared to $77.1 million for the nine months ended September 30, 2019. The $7.8 million, or 10.1%, increase in total interest income on a tax-equivalent basis was primarily due to organic growth in the loan portfolio.
Interest income on the investment securities portfolio on a fully-tax equivalent basis increased $567,000, or 8.8%, during the nine months ended September 30, 2020 compared to the nine months ended September 30, 2019, due to a $59.4 million, or 24.1%, increase in average balances between the periods, which was partially offset by a 43 basis point decrease in the aggregate portfolio yield.
Interest income on loans for the nine months ended September 30, 2020 was $77.3 million, compared to $69.7 million for the nine months ended September 30, 2019. The $7.6 million, or 10.9%, increase was primarily due to