growth, led by the communications and computer markets, reflecting increased adoption of higher-power chargers for mobile phones and tablets, and our increased market share in these applications. Revenues from the consumer market grew as a result of strong demand for consumer appliances, as well as increased market share in appliance applications. Revenues from the industrial category increased driven by growth in a broad range of applications including home-and-building automation, battery operated tools and broad-based industrial applications.
Our top ten customers, including distributors that resell to original equipment manufacturers, or OEMs, and merchant power supply manufacturers, accounted for 80% of net revenues in both the three and six months ended June 30, 2021, and 59% and 56% of net revenues in the corresponding periods of 2020. In the three months ended June 30, 2021, two customers, distributors of our products, accounted for 29% and 19% of our net revenues. In the six months ended June 30, 2021, the same customers accounted for 30% and 19% of our net revenues. In the three and six months ended June 30, 2020, one customer, a distributor of our products, accounted for 13% and 12% of our net revenues, respectively. International sales accounted for 98% of our net revenues in each of the three and six months ended June 30, 2021, and 97% of our net revenues in each of the corresponding periods of 2020.
Our gross margin was 50.7% and 50.1% in the three months ended June 30, 2021 and 2020, respectively, and 49.7% and 50.8% in the six months ended June 30, 2021 and 2020, respectively. The increase in gross margin in the three months ended June 30, 2021, as compared to the corresponding period of 2020, was due primarily to manufacturing efficiencies. Our gross margin decreased in the six-month period of 2021, as compared to the corresponding period of 2020, as improved manufacturing efficiencies only partially offset an unfavorable change in end-market mix as a greater amount of revenues came from lower-margin end markets.
Total operating expenses were $46.3 million and $40.6 million for the three months ended June 30, 2021 and 2020, respectively, and $90.6 million and $82.0 million for the six months ended June 30, 2021 and 2020, respectively. The increases in operating expenses for the three and six months ended June 30, 2021 as compared to the corresponding periods of 2020 were due primarily to higher salary and related expenses driven by increased headcount and annual merit increases, higher stock-based compensation expense related to performance-based awards, as well as increased materials engineering and equipment-related expenses in support of our product development efforts.
COVID-19 Pandemic
The COVID-19 pandemic has disrupted everyday life and markets worldwide, and governments around the world have imposed restrictions aimed at controlling the spread of the virus, including shelter-in-place orders, travel restrictions, business shutdowns and border closures. Beginning March 16, 2020 our San Jose headquarters location was subject to a shelter-in-place order, under which most of our employees were required to work from home; other locations around the world have also been subject to such restrictions. With restrictions lifting and high employee-vaccination rates, we have begun a phased reopening of our San Jose headquarters; with most employees expected to return in the coming months. Some of our employees in other locations around the world have also returned to the office under a phased reopening plan. We have implemented a variety of measures to protect the health and safety of our employees, including the provision of masks, gloves and sanitizers, social-distancing rules, and regular deep cleaning of our facilities.
While we have been able to conduct our day-to-day operations effectively in spite of the restrictions caused by the pandemic, in early 2020, the pandemic caused some disruptions in our supply chain. While our supply of wafers from our foundry partners was not interrupted, government-mandated closures in China, Malaysia, Sri Lanka and the Philippines caused temporary shutdowns at our assembly and test sub-contractors in those countries; all of the affected sub-contractors had resumed operations by the end of 2020. While these disruptions resulted in delayed shipments to some customers early in 2020, our results were not materially affected due to a variety of mitigation measures including higher-than-normal inventories of wafers and finished goods, safety stocks of certain key inputs, and multiple sources for components for most of our products. Although there are signs of improvement in many areas around the world, the potential for new lockdowns and other mitigation efforts to deal with an increase in infection rates in certain areas remains a key risk for our supply chain and the results of our business.
Despite the economic downturn stemming from the pandemic, demand for goods incorporating our products is strong. While the future trajectory of demand is uncertain, we believe our business is fundamentally sound with strong, long-term growth prospects. We have increased headcount and intend to continue investing in research and development and other functions necessary to support our future growth. We also intend to continue our cash dividend and stock-repurchase programs; however, if the economy deteriorates or our business outlook changes, our board of directors may