A tourist from Houston was in critical condition last night after being stabbed on the C train in a seemingly random act of violence, police officials said.

The stabbing happened in the last car of a southbound C train at 3:41 p.m. near the 110th Street station. The victim, whom sources identified as Christopher McCarthy, 21, entered the train with his girlfriend at 135th Street Station. The couple sat in the middle of the car.

As the train pulled into the 110th Street station, the unidentified assailant walked over and stabbed Mr. McCarthy in the chest with a sharp object. He then fled onto the platform. There was no attempted robbery or indication that the two men had prior contact, police said.

Mr. McCarthy first believed he had been punched hard to the chest, but soon realized the situation when blood began to wet his shirt, police said. Witnesses told police they saw the man make a punching motion at Mr. McCarthy before leaving the train. No weapon was recovered at the scene.

When Mr. McCarthy slumped back into his seat, bleeding, passengers notified the conductor through the emergency intercom. The conductor called the authorities and drove the train to the next stop, 103rd Street, where emergency workers entered the station and rushed Mr. McCarthy to St. Luke’s Hospital. He underwent surgery to save his life and as of press time had been upgraded to critical condition.

Detectives immediately began combing South Harlem and the Upper West Side for the assailant, who was identified as a 5-foot, 5-inch light-skinned black male in his 20s. He was dressed in black and wore a black cap, police said.

The couple was staying in Connecticut with a relative of the woman’s, sources said. They had come to Manhattan to go on a tour of Harlem.

The subway stabbing comes amid a sharp drop in crime in the subway system this year, with just 1,497 reports of crime versus 1,097 reports at the same time last year. Murders, however, are up 12.1% citywide this year.