French rider Remy Di Gregorio was being detained for questioning by French police, a gendarmerie source said on Tuesday.
Cofidis rider Di Gregorio, who was on the Tour de France, was transfered to his hometown of Marseille.
Di Gregorio is reportedly being questioned with two other people after an investigation was opened into alleged doping substances trafficking.
Di Gregorio, 26, who won a Paris-Nice stage last year, was regarded as the next great French climber when he started his professional career in 2005 but failed to live up to expectations.
In 2011, he joined Astana from FDJ before moving to Cofidis this season.
It was unclear whether Di Gregorio would be able to start Wednesday's 10th stage. Under French law, he can be detained for 48 hours.
Cofidis were not immediately available for comment.
