No charges have been filed, but a person of interest is being interviewed at a police precinct in Brooklyn.
The victim, 46-year-old Edwin Thomas, was driving a B-46 bus when the attacker boarded around noon Monday.
Police say the man swiped an invalid fare card and sat down. When Thomas denied him a transfer slip because he didn't pay for the ride, the man punched him twice in the head, stabbed him in the chest and torso, then fled.
Transit officials say it's the first on-duty slaying of a New York City bus driver in about 27 years.