Gemma Spofforth set a new world record in the 100m backstroke as she won Britain's first swimming gold in the pool at the World Championships.
The 21-year-old produced a stunning last 50m to overhaul Russia's Anastasia Zueva and win in 58.12 seconds.
Zueva, who lowered the world record in the semi-final, led by half a second at halfway but Spofforth finished strongly to win by six hundredths of a second.
Australia's Emily Seebohm was third and Britain's Lizze Simmonds was seventh.
It is the first victory in Rome by a swimmer using a part polyurethane suit as opposed to the others who have been wearing 100% polyurethane suits.
Gold 'a dream come true' - Spofforth
"I'm amazed. I just gave it all I had," Spofforth, who lives and trains in Florida, told BBC Sport.
"I am very excited with that result. I was thinking I have to take it out a bit faster than I did in the semi-final.
"Then I came home a bit faster."
Spofforth lost her mother, Lesley, to bowel cancer in 2007 and she dedicated the victory to her memory.
"My mother got me there over the last 15m," she said. "My mum's thoughts were always with me."
In the men's 100m backstroke Liam Tancock set a new British record in finishing joint fourth with Japan's Ryosuke Irie in 52.73.
Tancock set out fast and turned in second place but he faded down the final 50m as Japan's Junya Koga finished quickest to win in 52.26.
"I gave my all and just missed out on a medal but I'm pretty pleased with that," he told BBC Sport.
Joanne Jackson lowered her own British record by more than one second in qualifying in third place for Wednesday's final of the women's 200m freestyle.
The 22-year-old finished second behind America's Dana Vollmer in her semi-final in 1:55.54. Italy's Federica Pellegrini set a new world record of 1:53.67 to qualify quickest for the final.
"I was shocked with that time," Jackson told BBC Sport. "I now just want to get in there tomorrow and go quicker, put in a good performance and a good time and then we can have some fun in the (4x200m) relay on Friday."
Michael Rock smashed Steve Parry's British record in the semi-finals of the 200m butterfly.
His time of 1:54.58 knocked one second off the previous best to qualify sixth fastest for the final.
"I paced the race a lot better," he said. "It's my first major final so I'm really looking forward to Wednesday night.
"I'm just made up to get there and hopefully this is the first of many good things to happen over the next few years."
Spofforth's gold is Britain's third at the championships, following Keri-Anne Payne in the 10km open water swim last week and Tom Daley's 10m platform diving triumph.
Jackson and Rebecca Adlington won silver and bronze respectively in the 400m freestyle earlier in the week.
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