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

Phelps, winner of eight gold medals at the Beijing Games last August, won in 50.22 sec, 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."