Right after Phelps was done, 15-year-old Katie Ledecky -- the youngest member of the U.S. team -- nearly broke the world record to win gold in the 800 freestyle, denying Britain's Rebecca Adlington a repeat before her home fans. Adlington settled for bronze in a race Ledecky dominated from start to finish, falling off record pace only in the last 15 meters.
But no one has dominated like Phelps, who increased his career overall medal total to 21.
''He's the king of the Olympics Games,'' said his butterfly rival, Serbia's Milorad Cavic.
Even though Phelps didn't go as fast in the final as he did in the semifinals, he actually won by a relatively comfortable margin compared to his two previous Olympic wins in the 100 fly -- by four-hundredths of a second over Ian Crocker in 2004, then by one-hundredth of a second (the closest race possible) against Cavic at the Beijing Games four years ago.
That was the victory that kept Phelps on course to win a historic eight gold medals in China.
This was about going out in style.
Phelps touched in 51.21 seconds to beat out the guy who edged him in the 200 fly, Chad le Clos. The South African touched in 51.44, tying for silver with Russia's Evgeny Korotyshkin. Cavic tied for fourth in 51.81, not even close to Phelps in their final meeting.
''I cannot be compared to Michael Phelps,'' said Cavic, who also plans to retire after the London Games. ''I'm a one-trick pony.''
France won its fourth gold at the pool, its best showing ever, as Florent Manaudou -- younger brother of 2004 gold medalist Laure Manaudou -- shocked defending Olympic champion and world-record holder Cesar Cielo of Brazil in the 50 freestyle.
Manaudou touched in 21.34 and pounded the water out in Lane 7, then got a big hug from his sister as soon as he climbed from the pool. Cullen Jones of the United States took silver in 21.54, while Cielo was left with bronze in 21.59. Anthony Ervin, the 2000 Olympic champion who came back from an eight-year retirement, finished fifth after getting off to a poor start.
''I'm tired,'' said Cielo, still only 25 but the world's top sprinter for the last four years. ''Age takes its effect.''