An acoustic telemetry study was performed on young European sturgeon Acipenser sturio , an endangered species, to identify their movements in the Gironde estuary as well as their presence in zones which could not be sampled by trawling. Of particular interest was the influence of tidal and nycthemeral cycles on the fish. Between May and August 1999, 16 young sturgeons aged 4-5 years (total length: 82-122 cm; weight: 2.35-8.10 kg) were captured by trawl and fitted with coded ultrasonic transmitters. Search and monitoring campaigns were conducted 2-3 days a week until November 1999. The strategy was to monitor over a complete tidal cycle (flood-ebb). Sturgeon movements were mainly oriented to follow the tidal current direction, whether by day or by night. No significant behavioural differences were observed between daytime and night-time. Young sturgeons tend to congregate and stay in an area (a maximum of six fish were identified at a given moment) with an average depth of 7 m covering approximately 30 km2 in mid-estuary, where their favourite prey, polychaete worms, are very concentrated.