Ian Thorpe won a record ninth world championship gold medal with victory in the 400m freestyle for the third time in succession.
The 20-year-old Australian overtook the record he shared with East German Kornelia Ender - but his bid for seven gold medals in Barcelona came unstuck when Australia lost their 4x100m relay title to Russia.
Fellow Australian Grant Hackett, silver medallist behind Thorpe in 1998 and 2001, led narrowly for the first half of the 400m freestyle in the Palau Sant Jordi pool.
But Thorpe cut loose to overhaul his compatriot and win in three minutes 42.58 seconds, and become the first swimmer to win the same individual event at three world championships.
"Tracey (coach Tracey Menzies) told me to concentrate on the ABC, and I did, and make sure I stayed in control of the race," said Thorpe.
Hackett took the silver in 3:45.17 and Dragos Coman of Romania the bronze in 3:46.87.
"All in all it's not a bad start for me," said Hackett. "With the first gold going to Australia it's still a great start."
But Australia could not make it a second in the relay as Alexander Popov ended a 12-year wait for 4x100m freestyle gold with the Russian team.
Popov, who clinched his first world title since 1998, raced the final leg after teammates Andrei Kapralov, Ivan Usov and Denis Pimankov and clocked a new championship record of 3min 14.06sec, ahead of the United States in 3:14.80.
"It's a nice sort of forgotten feeling," said the 30-year-old.
"I've never been on the top of the podium for the 4x100m relay, so this is a pleasure. I've been dreaming about it since 1991."
And the four-time Olympic champion, who is competing in the 50m and 100m this week, warned his rivals: "This is only the beginning for me."
Australia finished out of the medals as France pipped them for bronze behind the United States.
"I'm a little bit disappointed with the overall result for the relay team. We've done what we've done in the past but there was a really hot field out there," said Thorpe.
Jenny Thompson equalled the most medals won at world championships as she anchored the USA to victory in the women's event.
The 30-year-old, who has six gold, three silver and a bronze medals, covered her 100m leg in a world record time of 53.44 seconds, but records can only be broken on the first leg of the relay.
"I don't know what it is but I just got very excited," she said.
"I came off the 50 turn and felt an adrenalin rush. I didn't see anyone around me and I just stayed very focused,."
Germany's Hannah Stockbauer won the women's 400m freestyle title.
Stockbauer, defending champion over 800m and 1,500m, came in ahead of Eva Risztov of Hungary with Diana Munz of the United States taking bronze.
Japan's Kosukke Kitajima, meanwhile, moved closer to the world record when he clocked 59.98sec to qualify for the men's 100m breaststroke final.