Crews were called to a building on Blackstone and Floradora around 11:15 p.m. Wednesday. When they arrive, smoke and fire was coming from both floors.
After the fire was under control, firefighters went inside, and that's when the second story floor gave way.
"There was a hole in the floor covered by some sheetrock from a previous fire and one of our fire captains stepped through the sheetrock and went half way through the floor to his chest," said Chuck Tobias, Fresno Fire Department.
The injured firefighter has not been identified. Officials say he received some injuries to his knee and torso, and was taken to Community Regional Medical Center to be checked out.
Investigators say the building is abandoned and the cause of the fire is still under investigation.