The flames broke out on the 4300 block of North 9th Street in Philadelphia's Hunting Park section around 4:35 a.m. Thursday.
Fire crews found heavy fire showing on the first floor. They had trouble accessing the building.
One apartment tenant was displaced. The American Red Cross of Southeastern Pennsylvania is currently on the scene to assist the resident.
Fire crews say it's possible an electrical hookup caused the fire, but that remains under investigation.
