Phelps completed the 400 individual medley in 4 minutes 8.41 seconds on Wednesday to really bury his old mark.
"It's a relief, I will say," Phelps, 19, said. "The Olympic trials is probably more stressful than the Olympic Games itself. Being able to come in tonight and get the first race under your belt, to be able to step back and relax a little bit."
Not only did Phelps' record get the crowd of 7,781 on its feet, it also sparked a strong showing by his colleagues.
Two American records followed Phelps' world-record performance: Klete Keller in the final of the 400 freestyle (3:44.19) and Brendan Hansen in the semifinals of the 100 breaststroke (1:00.13).
Second-place finishers had good times too. Erik Vendt finished behind Phelps in 4:14.09, while Larsen Jensen finished second to Keller in 3:46.56.
Another record almost fell, as Phelps' club teammate from North Baltimore, 15-year-old Katie Hoff, missed the American record by nine one-hundredths of a second, winning the 400 individual medley in 4:37.67. She knocked nearly 12 seconds off her time of the morning preliminaries.
And so, Phelps' symbolic scrawl of December turned out to be a star gesture in July. He must have known something months ago when the pool here was only a parking lot. After winning on Wednesday, he looked at the clock, spotted the time and pumped his fist. His rewrite job was complete.
It is the third time Phelps has broken the world record at that distance, and there is more symbolism. Phelps became the first U.S. Olympian on the 2004 swim team and this was the first time he had won an event at trials.
The final time didn't come as a shock. Some thought Phelps took a look at the clock during his backstroke leg, and he owned up to doing so.
"Oh yeah. If the clock is there I'm going to look," he said, smiling.
"It happens all the time. So I was probably about a quarter of a way down the second 50 of backstroke, I glanced up, and saw the negative six-tenths under world-record pace, or something."
It could be the first of many at the trials and he's almost blase about these records. Phelps broke five world records at the world championships in Barcelona, Spain, last summer, and is keeping some of the same Barcelona karma going, listening to the same music as he did then.
Why change? Phelps set the tone on what was a successful first day and night for his peers. Vendt, who finished second to Tom Dolan in the 2000 trials, was thrilled, especially since the 400 individual medley is not his favorite event.
"He's got a lot of confidence," said Vendt's coach, Mark Schubert. "He's got a lot more in him. It's a great way to start the week out, in the first event and to make the team. And now we can focus on the mile."
Schubert, who was also in Barcelona, was shaking his head about another spectacular performance by Phelps, who has entered six individual events at the trials, the first step along the road to chasing Mark Spitz's record seven gold medals at one Olympics.
"His first two strokes were just so awesome," Schubert said.
"He just has the ability to set it up really well. He has a lot of confidence and it's a great way to get the whole event started."
The other individual medley winner, Hoff, is from Phelps' swim team, the North Baltimore Aquatic Club.
That connection is a bit deceptive because they don't train together or even practice at the same complex. But there have been a steady line of 15-year-old prodigies from that club.
Hoff survived an emotional morning. She went out too fast in the preliminaries, qualified with a 4:49.16 and did not speak with reporters despite requests.
Her coach Paul Yetter was visibly tense, saying he told her, " 'You're sixth. Get ready for tonight.' "
She rallied and the morning gloom was replaced by evening glee. Kaitlin Sandeno of USC took second in 4:40.39, and Hoff nearly took out the venerable American record, 4:37.58, set by Summer Sanders in 1992.
"During the first 100, I was like, 'OK, OK, just keep to your plan,' " Hoff said.
"Our plan for the breaststroke was just to take off and try to pass some people. I was passing people, so I was like, 'Oh my gosh. I am really doing well here.' "
Gary Hall Jr. and Jason Lezak, who could meet twice at the trials in the 50 and the 100 freestyle, have been taking shots at one another.
Lezak sent a missive to Hall's Race Club and made fun of Hall's physique, saying, " ... You look a little soft. You must be enjoying your food these days."
Hall or his handlers shot back on the website by referring to Lezak as a "Professional Relay Swimmer."
Lezak acknowledged sending the e-mail and shook his head at the relay rip. "There haven't been any words, but it's not like he's my friend and I want to go hang out with him," he said.
But Hall's manager, David Arluck, didn't mind throwing a match onto the kindling, saying: "There's not a rivalry between him and Jason. It doesn't exist yet because Jason hasn't accomplished anything on an international level like Gary has.... It's like Spud Webb kicking Michael Jordan in the shins."
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For the Record
Swimming events in which Michael Phelps holds the world record:
* 200 butterfly (1:53.93 on July 22, 2003, at Barcelona, Spain)
* 200 individual medley (1:55.94 on Aug. 9, 2003, at College Park, Md.)
* 400 individual medley (4:08.41 on July 7, 2004, at Long Beach)
* 4 x 100 medley relay (3:31.54, with Aaron Peirsol, Ian Crocker and Jason Lezak, on July 27, 2003, at Barcelona)