Mike Tyson, continuing his quest to become the youngest heavyweight champion in boxing history, knocked out William Hosea at 2:03 of the first round tonight to score his 21st knockout and improve his professional record to 23-0.

Tyson, who turns 20 on Monday, dominated from the beginning, pounding Hosea with body blows and dropping his opponent with a right-left-right combination.

Hosea, 29, of Bloomington, Ill., got up and indicated he thought he had beaten the count, but referee Harry Papacharalambous declared a knockout over boos from the crowd of about 1,200 at Rensselaer Polytechnic Institute fieldhouse.

"At nine I thought my hands were off the canvas, and he the referee said 'It's over,' " said Hosea. "I came here to give Tyson a fight. If Tyson is going to knock me out, let him knock me out."

"He wasn't up at the count of 10," said Papacharalambous.

"I hurt him with at least two good body punches," Tyson said. "I felt his strength had left him."

It apparently was the first time Hosea had been knocked out, although there was some question about his record. He was billed as 17-3 coming in, and promoters said all three losses had been 10-round decisions and he never had been knocked down.

Tyson knocked out his first 19 opponents, 12 in the first round. He then won 10-round decisions over James (Quick) Tillis, his first world-class opponent, and Mitch Green, but this week was ranked the No. 2 contender by the World Boxing Council, behind champion Trevor Berbick and Pinklon Thomas.

During a 75-10 amateur career, Hosea defeated Green, knocked out former ranked heavyweight Renaldo Snipes and knocked down former champion Greg Page.

"That's why," Tyson's trainer, Kevin Rooney, said, "he could be Mike's toughest opponent."