Coeursamba delivered a surprise result at the Emirates Poule d'Essai des Pouliches at ParisLongchamp to deny Mother Earth a Classic double.
The Zoffany filly travelled strongly for much of the one-mile contest and briefly looked set to prevail after picking up well inside the final two furlongs.
The Aidan O'Brien-trained Mother Earth, a 6/4 favourite, was backed to follow up from its victory at the 1000 Guineas in Newmarket and it looked as if that would be the case.
However, she did not pick up as well as Jean-Claude Rouget's Coeursamba at the French racing equivalent, who was a 66/1 shot under Cristian Demuro.
The three-year-old had won just one of her seven previous starts, most recently finishing third in the Prix du Louvre over the course and distance last month but won the race in good fashion.
Rouget told Sky Sports Racing: "It's always a great pleasure to win a Group One.
"It's not easy, the race was very open and my filly showed very good things last year at the top level, but didn't win a Group race.
"She had a good rest in the stud all winter and we gave her a very smooth first race (of the year).
"Today she was on top, she had a good draw and she did the double acceleration I like for Group One horses.
"She's progressed a lot since the Prix du Louvre. When I saddled her today, I felt she had grown."
Coeursamba appears unlikely to bid for further Classic glory in the French Oaks, with Rouget keen to stick to a shorter distance - potentially in the Coronation Stakes at Royal Ascot.
He added: "I feel she is better at a mile.
"It (Coronation Stakes) is possible. She's not entered - I didn't enter any horse because of plenty of things.
"There is the Prix Rothschild or she can run in the Prix Jean Prat against the colts, or we could supplement (for the Coronation Stakes)."