They entered their pleas Friday before U.S. Magistrate Judge J. Bradford Stillman in Virginia.
Their trials are scheduled for July 6.
Eleven Somali men have been indicted on U.S. charges. One pleaded not guilty Wednesday and another is scheduled to be arraigned next week. All face mandatory life sentences if convicted.
The nine men were scheduled to enter pleas on Wednesday, but attorneys representing them said they needed more time to explain the government's accusations with an interpreter present. Some said they had spent only 30 minutes with their client and were not confident they understood U.S. court procedures.