In which event did Michael Phelps set his last individual world record?

Michael Phelps set his last individual world record in the 100m butterfly. He recorded 49.82 seconds in the semifinal at the 2009 World Aquatics Championships in Rome, lowering the mark he had set earlier that year.

The record came on August 1, 2009, during a championship in which Phelps wore a controversial polyurethane-era racing suit. He had already broken the 100m butterfly record at the U.S. national championships in July, swimming 50.22. In Rome, he became the first swimmer to break 50 seconds in the event, a landmark that made 49.82 one of the most famous performances of that period.

Phelps also set a world record in the 200m butterfly at the same championships, posting 1:51.51 in the final. That creates a common mix-up: the 200m butterfly was his signature event and produced another Rome record, but the 100m butterfly mark came later and was his final individual world record. He retired with individual records across several distances and strokes.