Rebecca Adlington has won Britain's first medal in the pool, winning bronze in the women's 400 metre freestyle final.
French swimmer Camille Muffat took the gold, beating the American favourite Alison Schmitt into second place.
The team GB swimmer had won gold in the event in Beijing, but actually swam faster this time around.
Adlington said "I'm so glad I got a medal at a home Games. Not many people can say that."
Rebecca becomes the first British woman to win swimming medals at successive Olympic Games.
Although she won two gold medals in 2008, not many people expected her to repeat that again in London.
She begins her defence of the 800m freestyle title in the heats on Thursday, with the final on Friday evening.
"The crowd was so overwhelming. Twelve years' hard work has gone into that," Adlington added.