LOS ANGELES — Shane Mosley had the speed, everyone knew that.

The surprise was his power.

Mosley dominated from start to finish, stopping Antonio Margarito at 43 seconds of the ninth round Saturday night to win the WBA welterweight title in a stunning upset.

The knockout was the first of Margarito's career.

"People underestimate my strength," Mosley said. "(Ricardo) Mayorga, (Fernando) Vargas, these guys are big. I'm able to knock them out. Margarito didn't feel nearly as big."

Margarito was coming off an impressive victory over previously unbeaten Miguel Cotto six months ago that had 30-year-old slugger being touted as one of the top pound-for-pound fighters.

Mosley must not have been paying attention.

The 37-year-old Mosley used his superior quickness from the beginning, and Margarito never was able to display the power and fortitude he showed against Cotto.

"It was a great night for me. Hopefully, there are many, many more," Mosley said. "I wasn't really expecting a knockout. It happened."

When asked what won the fight, Mosley replied: It was my strategy, my focus, my game plan. It t was a tough fight, but it was a great plan. It was my left hook. I caught Vargas with it, I caught Mayorga with it.

"Margarito's a warrior, he's going to win more belts. He's just a great guy, I'm happy he took the fight."

Mosley brought in Nazim Richardson to train him for this fight, replacing his father, Jack. That move worked to perfection.

"When you have a great game plan and an excellent athlete, then everything works out very well," Richardson said. "Shane Mosley's an excellent athlete. Shane Mosley's an excellent student."

The bout was held before an announced crowd of 20,820 — largest to attend a sporting event at Staples Center since it opened in October 1999. The fans were clearly pro-Margarito, even though Mosley grew up in suburban Pomona. It meant little once the bout began.

"I feel OK. I was just getting caught over and over," said Margarito, who was taken to a hospital to be checked out.

Margarito, of Tijuana, Mexico, didn't win a single round on one judge's scorecard, one on another and two on the third before the ninth. The Associated Press had Mosley every round but one.

Mosley, who weighed the maximum 147 pounds, raised his record to 46-5 with 39 knockouts. Margarito, who weighed 145.8 pounds, dropped to 37-6 with 27 knockouts.

Both fighters earned around $2.4 million.

While Margarito was at his best in beating Cotto, Mosley certainly wasn't in his last outing, a 12th-round knockout of Mayorga four months ago at Home Depot Center in nearby Carson. Mosley was ahead on two of the three judges' scorecards when the end came in that fight.

Mosley was far better against Margarito. Before beating Mayorga, Mosley lost a close but unanimous 12-round decision to Cotto, on Nov. 10, 2007, and said afterward he'd love another shot at Cotto.

Before the fight, HBO's Larry Merchant reported Margarito had to have his hands rewrapped after a complaint by Richardson during his pre-fight inspection.

Stephen Espinoza, attorney for Golden Boy Promotions, said he was told by Dean Lohuis, co-executive director of the California State Athletic Commission, that a plaster-like substance was found under both of Margarito's hand wraps and had been bagged as evidence.

None of that mattered once the action began. Mosley landed 178 punches to 108 for Margarito and 118 power punches to Margarito's 78.

Margarito fans at ringside, perhaps understanding their fighter was in trouble, began a pleading "Margarito, Margarito" chant as the eighth round began, and the Mexican boxer's corner came to life when he landed his most solid blows of the fight to that point.

But Mosley staggered the champion late in the round, and floored him with a barrage of punches as the round ended. Margarito wobbled to his feet to beat the count to 10, but looked like a beaten man as he sat in his corner.

Mosley continued to force the action in the ninth, and finally Margarito's corner threw in the towel when the fighter was unable to defend himself. Mosley landed 18 power punches and 21 overall to none for Margarito in the final round.

"Something happened in the first round," said Javier Capetillo, Margarito's trainer. "We were too slow. I didn't think he was reacting properly. It was frustrating to watch because he kept getting hit by overhand rights."

The fans began a "Margarito, Margarito" chant in the opening seconds of the first round, and the fighters were slugging it out toe-to-toe by the middle of the round. Mosley got what appeared to be some good shots in, but Margarito responded with a smile.

The crowd roared when the fighters traded solid punches early in the third round, and Margarito again flashed a grin after Mosley nailed him with several blows, none of which seemed to do much damage. But Mosley was connecting more often.

Margarito appeared to be picking up steam in the fourth round, but again, Mosley caught him with several punches, and Margarito didn't respond with a smile until Mosley nailed him with a right hand as the round ended. That grin appeared to be one of acknowledgment.

Mosley was fighting at Staples Center for the second time. He won a 12-round split decision over Oscar De La Hoya on June 17, 2000 in the first boxing match held at the downtown Los Angeles arena.

On the undercard, Robert Guerrero of Gilroy, Calif., stopped Edel Ruiz of Los Mochis, Mexico just 27 seconds into their 10-round featherweight bout.

Guerrero (23-1-1, 16 KOs) used a single left hand punch to the body to floor Ruiz, who spent a couple minutes on his hands and knees before being helped to his feet.