Average investments were $140.0 million for the quarter ended September 30, 2018 and $136.1 million for the quarter ended June 30, 2018. Interest income from the investment portfolio was $839,000 while average investments were $130.5 million for the quarter ended September 30, 2017. The tax equivalent yield on average investments for the quarter ended September 30, 2018 was 2.98%, down three basis points from 3.01% for the quarter ended June 30, 2018 and up three basis points from 2.95% for the same quarter in 2017.
Total interest expense was $705,000 for the three months ended September 30, 2018 and $573,000 for the same period ended June 30, 2018. This increase resulted mostly from the increased cost of interest bearing liabilities. The average cost of interest bearing liabilities increased 11 basis points when comparing the quarter ended September 30, 2018 to the quarter ended June 30, 2018. The average balance of interest bearing liabilities increased $5.6 million from the quarter ended June 30, 2018. The net interest margin was 4.04% for the quarter ended September 30, 2018 and 4.17% for the quarter ended June 30, 2018. For the quarter ended September 30, 2017, total interest expense was $351,000 and the net interest margin was 4.12%.
The Company’s net interest margin is not a measurement under accounting principles generally accepted in the United States, but it is a common measure used by the financial services industry to determine how profitably earning assets are funded. The Company’s net interest margin is calculated by dividing tax equivalent net interest income by total average earning assets. Tax equivalent net interest income is calculated by grossing up interest income for the amounts that arenon-taxable (i.e., municipal income) then subtracting interest expense. The tax rate utilized for periods in 2018 is 21% and 34% for periods in 2017.
Noninterest income was $1.8 million for the three months ended September 30, 2018 and $1.7 million for the quarter ended June 30, 2018. Noninterest income was $1.6 million for the quarter ended September 30, 2017. Nominal increases in ATM fees, safe deposit box fees and commissions fromnon-deposit investment sales collectively contributed to the increases between the quarter ended September 30, 2018 and the three-month periods ended June 30, 2018 and September 30, 2017.
Noninterest expense was $7.3 million for the quarter ended September 30, 2018. This represents an increase of $1.1 million or 18.55% from $6.2 million for the quarter ended June 30, 2018. Much of this increase resulted from the $987,000 loss accrued for the sale of other real estate owned. Salaries and employee benefits increased $259,000 from the quarter ended June 30, 2018 to September 30, 2018. This increase resulted mostly from an accrual made for anticipated employee incentive plan expense. Data processing fees increased $152,000 when comparing the quarter ended June 30, 2018 to the quarter ended September 30, 2018. Much of this increase is related to the Company moving itsin-house core banking software to a service bureau environment. The Company migrated to a service bureau environment in late June 2018. Noninterest expense increased $1.4 million or 23.71% from the quarter ended September 30, 2018 when compared to $5.9 million for the quartered ended September 30, 2017. This $1.4 million increase is also related to the increases in the previously mentioned expense categories.
Income tax expense decreased $576,000 when comparing the quarter ended September 30, 2018 to the quarter ended June 30, 2018. Income tax expense decreased $890,000 when comparing the quarter ended September 30, 2018 to the same period in 2017. These decreases resulted from the recognition of a $316,000 estimated tax credit related to the Company’s investment in a qualified affordable housing project. For additional details regarding the Company’s qualified affordable housing project investments, please see the Company’s Annual Report on Form10-K for the year ended December 31, 2017, and other filings with the Securities and Exchange Commission.
Asset Quality and Provision for Loan Losses
Nonperforming assets consist of loans 90 days past due and still accruing interest, nonaccrual loans, other real estate owned (foreclosed properties), and repossessed assets. Nonperforming assets decreased from $4.1 million or 0.53% of total assets at June 30, 2018 to $3.2 million or 0.40% of total assets at September 30, 2018. This decrease resulted from the recording of a $987,000 valuation allowance for other real estate owned during the quarter. During the third quarter of 2018, the Company entered into a contract to sell a residential property it held as other real estate owned. Based on the sales contract and estimated selling costs of the transaction, a loss of $987,000 had been accrued and added to the valuation allowance for other real estate owned. The sale is scheduled to settle on October 15, 2018. During the quarter ended September 30, 2018, two loans totaling $88,000 were placed on nonaccrual status. Most of thenon-accrual loans at September 30, 2018 are secured by real estate. Management regularly evaluates the financial condition of borrowers with loans onnon-accrual status and the value of any collateral on these loans. The results of these evaluations are used to estimate the amount of losses which may be realized on the disposition of thesenon-accrual loans. No real estate assets had been foreclosed upon during the third quarter of 2018. There were no loans greater than 90 days past due and still accruing at September 30 and June 30, 2018. At September 30, 2017, there were $19,000 in loans that were greater than 90 days past due and still accruing. Nonperforming assets were $5.2 million or 0.71% of total assets at September 30, 2017.
The Company may, under certain circumstances, restructure loans in troubled debt restructurings as a concession to a borrower when the borrower is experiencing financial distress. Formal, standardized loan restructuring programs are not utilized by the Company. Each