The man was swallowed up while he was in a collapsing bedroom of the house in Brandon, investigators said.
The hole was up to 100 feet wide and still growing Friday.
The house started to collapse when the sinkhole opened up under the house Thursday night and the man fell in despite his brother's efforts to save him, WPTV, Tampa, reported.
CNN reported that Hillsborough County Fire Department spokeswoman Jessica Damico said Friday, "It's not confirmed, but I think it's safe to presume [the victim is dead]."
The first law enforcement officer on the scene pulled the man's brother from the end of the chasm.
Authorities deemed the badly damaged house unsafe for rescuers, WPTV said.
Listening devices and cameras were placed in the gaping hole but there had been no contact with the missing man, WPTV said.
Crews said the rest of the building could collapse at any time.
WPTV said other homes in the area had been evacuated.