Aaron Peirsol broke his own world record as he powered to his third 200m backstroke world title in a row.
Peirsol has not lost in the 200m backstroke since the Sydney Games
The American touched in a time of one minute 54.66 seconds in Montreal to better the mark of 1:54.74 he set at last year's US Olympic trials.
Austrian Markus Rogan took silver and American Ryan Lochte bronze.
Australian Leisel Jones set the sixth world record of the championships as she took gold in the women's 200m breaststroke in 2:21.72.
Her time beat the previous record of 2:22.44 set by American Amanda Beard in last year's US Olympic trials by 0.72 seconds.
Germany's Anne Poleska was second in 2:25.84 and Austrian Mirna Jukic third in 2:27.11.
Peirsol, who had already successfully defended his 100m backstroke crown, has not been beaten in the 200m back since the 2000 Sydney Olympics.
He joins Australian freestylers Ian Thorpe and Grant Hackett as the only swimmers to win the same event at three successive world championships.
Michael Phelps claimed his fourth gold of the championships as the US team won the men's 4x200m freestyle relay final.
Phelps, Ryan Lochte, Peter Vanderkaay and Klete Keller clocked 7:06.58, with Canada taking silver and Australia bronze.
There was another gold for the United States as Brendan Hansen added to his 100m breaststroke title with victory in the 200m breaststroke final.
Hansen won in a time of 2:09.85, with Canada's Mike Brown second in 2:11.22 and Japan's Genki Imamura third in 2:11.54.
Australia's triple Olympic gold medallist Jodie Henry won the women's 100m freestyle title, storming to victory in 54.18secs.
European champion Malia Metella of France and Olympic backstroke gold medallist Natalie Coughlin of the United States tied for second in 54.74secs.