Russian Alexandr Kolobnev of the Katusha team withdrew from the Tour de France on Monday after testing positive for a banned substance.
The International Cycling Union (UCI) said a urine sample the 30-year-old Kolobnev provided on July 6 was found to contain traces of Hydrochlorothiazide.
The Russian is the first rider to fail a doping test during this year's Tour which started on July 2.
"Earlier today, the UCI advised the Russian rider Alexandr Kolobnev of an Adverse Analytical Finding (presence of Hydrochlorothiazide based on the report from the WADA accredited laboratory in Chatenay-Malabry) in the urine sample collected from him at an in competition test at the Tour de France on 6 July 2011," the UCI said in a statement.
Hydrochlorothiazide, often used in the treatment of hypertension and congestive heart failure, removes excess fluid from the body and reduces urine output.
Kolobnev has four days to request an analysis of his B sample.
"Team Katusha rider Alexandr Kolobnev, after testing positive for a diuretic at a medical examination during the Tour de France's first week, decided to suspend himself according to UCI rules, waiting for the B-sample," Katusha said in a statement.
"At the moment, Team management and the rider have no further comment."
Katusha added in the statement that team rules state if a B sample also test positive a rider will be sacked and have to pay "five times his salary" as a fine.
Kolobnev was lying 69th in the overall standings, 22 minutes and 15 seconds behind leader Thomas Voeckler.