America's Shane Mosley dominated from start to finish, knocking out Antonio Margarito of Mexico for the first time in his career in the ninth round Saturday to win the WBA welterweight title in a stunning upset.

Margarito was coming off an impressive victory over previously unbeaten Miguel Cotto six months ago, and boxing experts had dubbed the 30-year-old one of the top pound-for-pound fighters in the world.

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

"It was my strategy, my focus, my game plan," Mosley said when asked what won the fight. It was a tough fight, but it was a great plan. It was my left hook. I caught (Fernando) Vargas with it, I caught (Richardo) Mayorga with it.

"He's a tough fighter, he had a lot of endurance. I prepared very hard, I trained hard. He was very powerful, but he couldn't resist my rhythm."

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

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

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 despite the fact that Mosley grew up in suburban Pomona, but it meant little once the bout began.

"I feel OK. I was just getting caught over and over," Margarito said.

Margarito, 30, didn't win a single round on one judge's scorecard, one on another and two on the third. The Associated Press had Mosley every round but one.

Mosley raised his record to 46-5 with 39 knockouts. Margarito, known as the "Tijuana Tornado," dropped to 37-6 with 27 knockouts after being stopped for the first time.

Both fighters earned around $2.4 million from the bout.

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, California. Mosley was ahead on two of the three judges' scorecards when the end came in that fight.

Mosley was far better against Margarito. Mosley lost a close but unanimous 12-round decision to Cotto before that, 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, using his superior quickness, 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.