An explanation of the non-GAAP measure of gross sales and certain expenditures, which may be mentioned during the course of this call, is provided in the notes designated with asterisks in the condensed consolidated statements of income and other information attached to the earnings release dated August 8, 2012. A copy of this information is also available on our website, at www.monsterbevcorp.com, in the Financial Information section.
While the beverage industry, in general, experienced softness in second quarter sales volumes in North America and Europe, the energy category in our principal markets in the United States grew in the high teens in the second quarter, slightly lower than the growth rate of the energy category in the United States in the first quarter.
Overall, the company had a good second quarter with record gross sales up 28.7% to $678.9 million. Net sales up 28.2% to $592.6 million and operating income up 28.1% to $169.8 million. Diluted earnings per share increased 31% from $0.45 per share in the second quarter of 2011 to $0.59 per share.
We also recently commenced reporting sales information for the United States market based on an expanded sample of outlets, which now includes Walmart; dollar stores, such as Family Dollar, Dollar General and Fred's; DeCA military stores and club stores namely Sam's and BJ's, but excluding Costco. On this call and in the future, when we discuss the Nielsen reports for the United States for all outlets combined, the reports and information contained therein will be based on this expanded sample. This will not affect the reports and information covering sales in the convenience and gas channel, the grocery and or drug channels which may, from time to time, be referred to individually. As this will be the first time that we referred to reports based on the expanded sample, we will provide figures on both basis.