We had cost of revenues of $1.0 million for the three months ended June 30, 2021, an increase of $0.3 million, or 42%, compared to $0.7 million in the same period of the prior year. The primary reasons for this change were increased revenue and headcount. As a percentage of revenues, cost of revenues decreased to 7% in the second quarter of 2021, as compared to 11% in the prior year period.
Engineering and product development expense
We had engineering and product development expense of $1.0 million for the three months ended June 30, 2021, an increase of $0.2 million, or 24%, compared to $0.8 million in the same period of the prior year. The increase was primarily due to increased clinical studies costs as well as increased consulting costs associated with projects. As a percentage of revenues, engineering and product development expense was 7% in the second quarter of 2021, as compared to 12% in the prior year period.
Sales and marketing expense
We had sales and marketing expense of $3.6 million for the three months ended June 30, 2021, an increase of $1.1 million, or 47%, compared to $2.5 million in the same period of the prior year. The increase was primarily due to increased headcount and associated expense to serve a continued expansion of customer activities. As a percentage of revenues, sales and marketing expense decreased to 25% in the second quarter of 2021, as compared to 39% in the prior year period.
General and administrative expense
We had general and administrative expense of $2.3 million for the three months ended June 30, 2021, an increase of $0.8 million, or 54%, compared to $1.5 million in the same period of the prior year. The increase was primarily due to the growth in our business, which led to increased expenses including the expansion of board of directors, insurance and other professional fees. As a percentage of revenues, general and administrative expense decreased to 16% in the second quarter of 2021, as compared to 23% in the prior year period.
Other income/expense
We had other income of $9,000 for the three months ended June 30, 2021, compared to other income of $30,000 in the same period of the prior year. The decrease was primarily due to lower miscellaneous income and interest income.
Income tax benefit
We had income tax benefit of $215,000 and $85,000, respectively, for the three months ended June 30, 2021 and 2020. The tax benefit was primarily due to state income taxes (net of federal benefit), tax benefits associated with share-based compensation plans, and federal and state research and development, or R&D credit benefit.
Net income
For the foregoing reasons, we had net income of $6.7 million, or $1.00 per basic share and $0.83 per diluted share, for the three months ended June 30, 2021, an increase of $5.6 million, or 522%, compared to a net income of $1.1 million, or $0.16 per basic share and $0.13 per diluted share, for the same period of the prior year.
Six Months Ended June 30, 2021 Compared to Six Months Ended June 30, 2020
Revenues
We had revenues of $27.5 million for the six months ended June 30, 2021, an increase of $11.7 million, or 74%, compared to $15.8 million in the same period in 2020. Our revenues are primarily from fees charged to customers for use of our vascular testing products and from sales of accessories used with these products. We recognized revenues of $27.0 million from fees for our vascular testing products for the six months ended June 30, 2021, consisting of $14.8 million from fixed-fee licenses and $12.2 million from variable-fee licenses, compared to $15.4 million in the same period of the prior year, consisting of $12.4 million from fixed-fee licenses and $3.0 million from variable-fee licenses. The remainder was from sales of other products, which were $0.5 million compared to $0.4