net sales for the three months ended June 30, 2019 from 77.6 percent of net sales for the comparable period in 2018, primarily due to a favorable model mix, partially offset by increased labor cost.
Selling, general and administrative expenses for the three months ended June 30, 2019 were $9.0 million compared to $8.3 million for the comparable period in 2018, an increase of $0.7 million or 8.7 percent. This increase was primarily due to expenses that increase due to expenses that increase with higher sales and profitability, such as incentive compensation, as well as higher research and development expenses that support new product development. Selling, general and administrative expenses as a percentage of net sales increased to 10.2 percent in the second quarter of 2019 from 9.6 percent in the second quarter of 2018 primarily due to increased labor cost.
Operating income for the three months ended June 30, 2019 increased $0.2 million or 2.0 percent compared to the same period in 2018 primarily due to an increase in gross profit, partially offset by an increase in selling, general and administrative expenses.
Interest income for the three months ended June 30, 2019 increased $10 thousand or 11.8 percent compared to the prior year primarily due to a higher return.
Income tax provision for the second quarter of 2019 reflects an effective tax rate of 18.3 percent, compared to an effective tax rate of 20.0 percent for the comparable period in the prior year. The slight decrease in effective rate in the second quarter of 2019 is primarily due to higher tax exempt investment gains in 2019 as compared to the same period in 2018. The effective rate in both periods includes the effect of beneficial permanent differences including discrete adjustments related to restricted stock dividends and liabilities related to state income taxes.
SIX MONTHS ENDED JUNE 30, 2019 COMPARED TO SIX MONTHS ENDED JUNE 30, 2018
Net sales for the six months ended June 30, 2019 increased $7.2 million or 4.4 percent compared to the same period in 2018. The change in net sales during the quarter compared to the prior year was due primarily to a 12.1 percent increase in the average selling price per boat partially offset by a 7.2 percent decrease in the number of units sold. The increase in the average selling price per boat is primarily due to a model mix which included larger boats, such as our new 300 OSX Bowrider, as well as several of our larger Robalo models. The decrease in unit sales was primarily due to a decrease in sales of our smaller boats. During the six months ended June 30, 2019, net sales outside of the United States accounted for 6.8 percent of net sales compared to 9.3 percent of net sales during the comparable period of 2018. International sales remain low primarily due to the recently imposed tariffs on boat imports by Canada, Mexico and the European Union, coupled with the strong U.S. dollar. Domestic net sales increased 7.3 percent to $160.1 million but international sales decreased 23.8 percent to $11.6 million compared to the first six months of the prior year.
Cost of goods sold for the six months ended June 30, 2019 was $132.6 million compared to $127.4 million for the comparable period in 2018, an increase of $5.2 million or 4.1 percent. Cost of goods sold decreased slightly to 77.2 percent of net sales for the six months ended June 30, 2019 from 77.4 percent of net sales for the comparable period in 2018, primarily due to a favorable model mix, partially offset by increased labor cost.
Selling, general and administrative expenses for the six months ended June 30, 2019 were $18.9 million compared to $16.9 million for the comparable period in 2018, an increase of $2.0 million or 11.4 percent. This increase was primarily due to expenses that increase due to higher sales, as well as higher research and development expenses and an increase in labor cost. Selling, general and administrative expenses as a percentage of net sales increased to 11.0 percent in the six months ended June 30, 2019 from 10.3 percent in the six months ended June 30, 2018 primarily due to an increase in labor cost as well as increased research and development expenses to support new product development.
Operating income for the six months ended June 30, 2019 increased $0.1 million or 0.3 percent compared to the same period in 2018 primarily due to an increase in gross profit, partially offset by an increase in selling, general and administrative expense.