WorldCom, now known as MCI, and six of its former executives, including chief executive Bernie Ebbers, are charged with defrauding Oklahoma investors by inflating revenues and understating expenses.
The company could be ordered to pay millions in fines and restitution.
Carol Petren, MCI deputy general counsel, issued a statement criticizing Oklahoma Attorney General Drew Edmondson for bringing the case.
"It is extremely disheartening that Mr. Edmondson has brought charges against a company given the dramatic steps MCI has taken to put its house in order," she said.
She said the company has a new management team and internal controls in place to prevent a repeat of the problems that led to WorldCom's downfall in the biggest bankruptcy in U.S. corporate history.
Last week, Ebbers pleaded innocent to defrauding Oklahoma investors and was released on $50,000 bail. Each of the 15 charges carries 10 years in prison and a $10,000 fine.
The Oklahoma charges are the first criminal counts against Ebbers and against the company itself, though other former WorldCom executives have been charged in federal court.