Lithuania's Ruta Meilutyte added the 100m breaststroke world title to her Olympic gold with victory in Barcelona.
The 16-year-old, who is based in Plymouth, clocked one minute 4.42 seconds, just outside the
"Being a part of history is absolutely amazing," said Meilutyte.
United States teenager Katie Ledecky won the 1500m, knocking six seconds off the world record to finish in 15.36.53, ahead of Denmark's Lotte Friis.
"It was really tough, my hardest race ever," admitted the American, who bettered compatriot Kate Ziegler's previous mark.
After winning 400m gold on Sunday, Ledecky will now target a historic treble in the 800m freestyle on Saturday.
Meilutyte, who has attained superstar status back in her homeland since winning Olympic gold last summer, was cheered on by a number of Lithuanian fans who had made the trip to Spain.
"One of my biggest reasons I do this is to bring more people to swimming," she said.
"There are one or two more people in the crowd now and I'm really proud to be Lithuanian and have fans here."
Ledecky was locked in a battle with Friis, Rebecca Adlington's old rival, throughout their 1500m final and only pulled clear in the final 150m. New Zealand's Lauren Boyle finished third.
"I knew that we were going pretty fast and I just figured that whoever was going to come out on top would probably get the world record," she said.
"I just had to be careful not to push it too early or leave it too late."
Great Britain's Robbie Renwick was sixth in the 200m freestyle final in a time of 1:46.52, 12 months to the day after the Scot finished in the same position at the London Olympics.
The race was won by Frenchman Yannick Agnel, with American Conor Dwyer and Danila Izotov of Russia filling the other podium positions.
"It was a good swim for me apart from coming sixth, which is a disappointment because it's all about medals," said Renwick.
"I would have loved to have stepped it up [from last year] but I was a bit short and there are a few things I know I can work on before the Commonwealths."
Team-mate Roberto Pavoni missed out on the 200m butterfly final after finishing with the slowest time in the semi-finals. But he will return for his favoured 200m and 400m individual medley events later in the week.
"I'm pretty disappointed," said the Loughborough-based swimmer. "I really wanted to go quicker tonight and prove to everyone I'm a top-class butterfly swimmer, but I didn't."
In the night's other finals, US Olympic champion Missy Franklin won her first individual title in Barcelona with victory in the 100m backstroke, adding to the 4x100m freestyle gold she won on Sunday.
Australia's Emily Seebohm took silver ahead of Aya Terakawa of Japan.
Matt Grevers, who won gold at London 2012, won the men's event, with US team-mate David Plummer second and defending champion Jeremy Stravius of France in third.