U.S. regulators have begun a formal probe into stock option practices at Activision, the No. 2 U.S. video game publisher, according to a company filing.

Activision, whose hit titles includes "Guitar Hero 2" and "Spider-Man 3," learned this month that the Securities and Exchange Commission had issued a formal order of nonpublic investigation, which allows the agency to subpoena witnesses and require the production of documents, according to the filing on Thursday.