Seawater tolerance of Atlantic salmon (Salmo salar) smolts acclimated at 8 o C in fresh water was tested in seawater challenge tests at 2 o C, 5 o C, 11 o C, 14 o C and 17 o C, and compared to a test at 8 o C. Two tests were used, a 24-h, 35%% and a 24-h, 40%% salinity test. In 35%% salt water, there were no differences in plasma Cl - concentrations in the range 5-14 o C (mean values 140-144 mM). An increase in plasma Cl - level was found at 2 o C and 17 o C (mean values about 155 mM). The mean Cl - concentration at 8 o C in 40%% test was 10 mM higher than that at the same temperature in 35%% salinity. An increase in plasma Cl - level was found when the temperature shifted from 8 o C to either 5 o C or 14 o C (mean values 171 mM), and an even larger increase was recorded at 2 o C (mean value 209 mM, 75% mortality) and 17 o C (mean value 194 mM).The results indicated that smolt had the ability to hypoosmoregulate over a wide range of temperatures when transferred to full-strength sea water. However, differences from the acclimation temperature larger than 4-6 o C are not recommended, especially when the seawater temperature is lower than in fresh water. The use of a higher salinity than that of full-strength sea water may give a more comprehensive picture of the hypoosmoregulatory ability of smolts, and may possibly be used to distinguish differences in hypoosmoregulatory capacity of smolts not readily obtained from tests in full-strength sea water.