The Michael Phelps aquatic express, bound for the Olympics from Baltimore County, got sidetracked here Monday evening by a lanky California backstroker.

Phelps, 19, the nation's most celebrated swimmer who is shooting for seven gold medals in Athens, was beaten by Aaron Peirsol, 21, of Irvine, in a dramatic world-record-breaking 200-meter backstroke race on the sixth day of U.S. Olympic swimming trials.

Before Monday night, it had been a spectacular six days here for the gifted Phelps, who was unbeaten as he swam in glorious, seaside weather in a temporary outdoor pool before packed houses of almost 10,000 every night.

He had opened the trials last week by breaking his own world record in the 400 individual medley, claiming the first spot on the U.S. Olympic team. He received his first-place medallion in the 200 butterfly from Mark Spitz, whose record of seven gold medals Phelps is trying to match. And he served notice that he might be the American juggernaut at the Olympic games next month in Athens.

But Peirsol was the world record holder in the 200 backstroke. And even though Phelps had almost taken the record back in February, Peirsol was determined to keep it. He said Monday after the race that he realized he had become attached to it.

"Owning a world record, in just a little bit of time, kind of grows on you," he said. "When someone comes so close to . . . [taking] it, you realize how much it means to you. . . . It becomes your baby. You want to take care of it."

Phelps will still roll into Athens in fine style, swimming multiple events and relays. "He's a very tough guy, a very good guy," Peirsol said. But although Phelps also qualified for Athens in the 200 backstroke, Peirsol will again be in his way.

Monday night Peirsol finished the race in a time of 1 minute 54.74 seconds, breaking his own record of 1:55.15. He jubilantly hoisted himself up on one of the floating lane dividers and pumped one arm in the air in victory. Phelps was more than half a body length behind, finishing in 1:55.86.

Still, Phelps became the first American male to qualify for five individual Olympic events. And he had set himself an unprecedented task for the day, and for the entire eight-day trials. He entered six events in the meet, and already had raced 13 times, without a loss, since the trials began last Wednesday.

Monday morning he competed in, and won, a 100 butterfly heat. Then, starting at about 5:30 p.m., he raced Peirsol. Thirty minutes later he won the final of the 200 individual medley, one of three events in which he holds a world record. Then, about 30 minutes after that he won a semifinal heat in the 100 butterfly.

"I'm happy it's over," he said, adding that it was the toughest night of his career. "It's good to get that out of the way."

He was a little evasive about whether the loss would change his plans for Athens. He has been offered a $1 million bonus by Speedo, the swimwear company, if he wins seven gold medals.

"I honestly do feel pretty strong right now," he said. "I thought I would be more tired than I am." But he added, "We're going to have to see how things are going in the training camp to decide our final program."

Phelps's coach, Bob Bowman, had said earlier in the day that Phelps would have to "race his eyeballs" out against Peirsol. He noted that Phelps was in top, but perhaps not yet perfect, shape.

Peirsol's coach, Eddie Reese, who is also the coach of the men's U.S. Olympic swimming team, had said he believed both Phelps and Peirsol would go under Peirsol's two-year-old world record.

But only Peirsol made it. Peirsol already had qualified for the team in the 100 backstroke.

Peirsol, who is as long and thin as a plank, said he knew Phelps was "gunning" for him, and was determined to fend him off. After the race, his mother and stepfather, Wella and Tim Hartig, of Tustin, Calif., embraced, weeping, in the stands. "He's the king of the backstroke," his mother said.

Peirsol had first set the world record in the 200 backstroke in March 2002. The versatile and ambitious Phelps began to encroach on his turf in February when he swam the event in 1:55.30 at a meet in Orlando.

In May, Peirsol bested Phelps during a meet in Santa Clara, Calif., finishing first in a time of 1:56.95 seconds, to Phelps's second-place finish in 1:57.58.

Both said then they were happy with the results. Phelps said he was in the midst of his training program and hadn't peaked. Peirsol, who had suffered a minor bout with mononucleosis over the winter, said he still hadn't been feeling well.

Peirsol came back the next night and beat Phelps again, soundly, in the 100 backstroke.

Peirsol said last week that the wins heartened him. "It kind of gave me actually a little boost. It wears down on you: When people are saying, 'Michael's coming after you,' and all this kind of stuff. You start believing them almost. Then, finally . . . you catch yourself, you go, 'Okay, he's human like everybody else.' "

In the first of Phelps's four races Monday, he clocked a time of 53.27 seconds in winning his heat in the 100 butterfly.

But it was second to that of another archrival in the pool, Ian Crocker, 21, a native of Portland, Maine, who narrowly beat out Phelps for the world record of 50.98 seconds last summer. Phelps was stung by the defeat and has had a magazine photo of Crocker hanging in his bedroom as motivation ever since.

Crocker's time Monday morning was a quick 51.69 seconds. Phelps beat Crocker in the event May 22 during a meet in Santa Clara. The two face off in the final of the event Tuesday evening.

Michael Phelps swims toward his first-place finish in the men's 200 meter individual medley at the U.S. Olympic swim trials.Michael Phelps, 19, checks the official clock after finishing first in the men's 200 meter individual medley, one of the six events he entered at the trials.Aaron Peirsol, left, celebrates next to Michael Phelps after breaking the world record in the men's 200 meter backstroke during qualifying.