Bratz doll manufacturer MGA Entertainment Inc. has turned to state court to revive its legal battle against competitor Mattel Inc., filing a $1 billion trade secrets lawsuit on Monday.

The filing in Los Angeles County, Calif., Superior Court, was expected—MGA chief executive officer Isaac Larian promised the move last month after U.S. District Judge David Carter refused to let the company amend its federal lawsuit against Mattel over claims that the Barbie maker stole its trade secrets.