Google’s Form 10-Q filing with the Securities and Exchange Commission, made public this afternoon, contains an unexpected $500 million charge for the first quarter that wasn’t reported in the company’s quarterly results.

The filing says the charge was taken this month, and applied to the quarter ended March 31, “in connection with a potential resolution of an investigation by the United States Department of Justice into the use of Google advertising by certain advertisers.”

On the bottom line, the effect is to lower Google’s net income to about $1.8 billion, from the previously reported total of $2.3 billion, according to financial statements included in the SEC filing. Its diluted earnings per share similarly drop to $5.51 from $7.04 as a result of the charge, according to the filing.

It’s characterized in the filing as an event that occurred subsequent to the close of the quarter. Here’s the full text of the financial note that describes the charge.

In May 2011, in connection with a potential resolution of an investigation by the United States Department of Justice into the use of Google advertising by certain advertisers, we accrued $500 million for the three month period ended March 31, 2011. Although we cannot predict the ultimate outcome of this matter, we believe it will not have a material adverse effect on our business, consolidated financial position, results of operations, or cash flows.

A Google spokesman declined to comment, describing the issue as a legal matter.