Abstract The movement of sea trout.Salmo truttaL., smolts through the estuary of the River Avon, southern England was studied using miniature 300 kHz acoustic transmitters and an array of strategically placed acoustic sonar buoys. Sea trout smolts were trapped 3.25 km above the head of tide as they moved downstream and the transmitters were surgically implanted into the peritoneal cavity of the fish. Subsequent movement of the smolts through the estuary was predominantly nocturnal and occurred mainly during an ebb tide or the period of slack water between two high waters. During the hours of darkness the ground speed of the smolts through the estuary was similar to that of passively drifting objects. However, where movement did occur during daylight hours ground speeds of smolts were significantly slower. Vertical salinity profile measurements of the estuary indicated that individual smolts may have experienced salinities ranging from fresh to 100% sea water during their migration. The behaviour of the smolts is discussed in relation to the possible environmental and physiological cues controlling the estuarine migratio