Indianapolis, July 10

Michael Phelps broke the 100m butterfly world record, evidence that his post-Olympic transition is right on track.

Phelps, winner of eight gold medals at the Beijing Games last August, won in 50.22sec, slicing 18-hundredths of a second off the previous record of 50.40 set by compatriot Ian Crocker at the 2005 World Championships in Montreal.

“It was big,” Phelps said of the achievement. “This is something I really wanted to accomplish.” The performance took Phelps’s current tally of individual world records to five, to go with three relay world records. Since 2001, when he set his first world record in the 200m butterfly, Phelps has set at least one world record in all but one year — 2005.

In fact, Phelps has now qualified for three events at the upcoming World Championships in Rome, after victories in the 200m free and 200m butterfly on Wednesday. He pulled out of the 100m freestyle on Friday. Neither of his 200m victories here was up to his exacting standards, and when he saw his time on Thursday Phelps ripped off his cap in celebration as spectators at the Indianapolis University Natatorium rose and cheered.

The fact that the record had stood for four years, and was held by his friend and intense rival Crocker, made the performance even more satisfying.

Phelps managed to seize the world record in 2003 — for a day — lowering the mark in the semi-finals of the World Championships in Barcelona before Crocker responded by reclaiming it in the final. Phelps, who bested Crocker in the 2004 Olympic final, was stung by Crocker in the 2005 World Championship final.

Phelps knew he was closing in on Crocker’s mark when he swam 50.40 at a meeting in Montreal last month. But he admitted that he was a little surprised to be swimming so fast after his lengthy post-Beijing break. In addition, he returned to a new training regimen, which was promptly disrupted by a three-month competition ban over a tabloid photo of him with a marijuana pipe.