Michael Phelps wins 18th Olympic gold in final swim at 2012 Games
Phelps, 27, took the lead away from Japan during his leg of the relay with the butterfly, and Adrian stretched it to a 1.91 second margin with the freestyle.
The U.S. men won gold with a time of 3:29.35. Japan won the silver and Australia took the bronze. Phelps' own medal count is now a record 22 for his career, with 18 gold medals, two silver and two bronze.