A 22-year-old man was slashed in the back on a Manhattan train late Sunday, authorities said. 

The victim was arguing with another male straphanger on a northbound C train at the 81st Street-Museum of Natural History station on the Upper West Side around 11:45 p.m. when the suspect suddenly slashed him in the back, cops said. 

The attacker stayed on the train after the attack and was still on the loose early Monday, police said. 

The victim refused medical attention at the scene, authorities said. 

The nature of the dispute leading up to the assault is unclear.