Groups of Atlantic salmon (Salmo salar) smolts were exposed to full-strength seawater for 1, 6, 12, 24, 48, 96, and 144 h or were left in fresh water (control). Their physiological performance was monitored as changes in levels of plasma chloride, hematocrit, and muscle water content. At 12 and 24 h, smolts showed a transient increase in plasma chloride levels concurrent with a reduction in hematocrit, indicating a significant osmotic stress during this period. After 48 h, values fell to normal resting levels in seawater and no further changes were observed. In a second experiment, similar groups of smolts were exposed to seawater for 0 (control), 12 , 24, 48, and 96 h and then confronted with predatory cod (Gadus morhua). Groups exposed to seawater for 12 and 24 h showed a change in antipredator behaviour, i.e., reduced escape distance and schooling behaviour, and suffered significantly higher predatory mortality than control fish or groups fully acclimated to seawater. These results suggest that Atlantic salmon smolts that suffer physiological stress while transferring from fresh water to the sea may also be exposed to increased predation as a consequence of less effective antipredator behaviour. Resume : Des groupes de smolts de saumon atlantique (Salmo salar) ont OtO exposOs ‡ de lieau de mer ‡ pleine concentration