Thomas Voeckler seized the lead of the Tour de France after Stuart O'Grady won an incident-packed stage five.
The French champion, who began the day in 59th place overall, was fourth in the same time as O'Grady, who won a sprint finish involving five riders.
Five-times winner Lance Armstrong, who had led by three minutes, dropped to sixth overall, nine minutes behind.
The stage witnessed a number of crashes as bad weather and tricky road conditions made life very difficult.
Voeckler, who won the French title the week before the Tour on his 25th birthday, was part of a five-man breakaway which led for more than 185km of the 200km stage.
"I'm not really taking this in," said Voeckler after the race.
"I've dreamed about the yellow jersey before, but I didn't actually think I would pull it on.
"It's a lot of responsibility but I think I can handle it and I hope to keep it a few more days yet."
Armstrong, who took the overall lead after Wednesday's team time-trial, had said he would not defend his yellow jersey so early in the Tour.
"Tactically it's a great move for us with a Brioches (rider) in the yellow jersey," he said.
"Voeckler is a good young rider, he's French and I think it's a good thing.
"The team was great today. I kept telling them they wouldn't have to work for a week and so they were probably happy to hear that."
However, he would have been alarmed after seeing four US Postal riders crash in a massive pile-up halfway through the stage.
The crash also involved Italian sprint ace Alessandro Petacchi and second-stage winner Robbie McEwen - but Armstrong escaped unscathed.
O'Grady, who took his second Tour stage win - six years after the first in Grenoble in 1998 - dedicated the win to fellow riders David Millar and Matthew White.
Millar was banned from the race after admitting taking a banned drug, while White crashed and fractured a collarbone just hours before he was to have started the Tour.
"I said if I won a stage, when I won a stage, in this Tour, it was for two really good friends of mine, Dave Millar and Matt White," said the Australian.
"But it's also for everyone at Cofidis because it has been one hell of a ride."