The effect of FinnStim, a betaine/amino acid mixture, on growth and seawater adaptation was studied in yearling chinook salmon Oncorhynchus tshawytscha. The fish were marked with PIT tags and divided into two groups. One group was fed a diet with 1% FinnStim added and the other was fed a control diet with 1% α-cellulose. The diets were fed for 6 weeks in fresh water at a level of 1% body weight per day and for 8 weeks in seawater netpens at 3% body weight per day. Seawater challenge tests were conducted three times during the freshwater phase of the experiment. Growth and mortality in fresh water did not differ significantly between treatments. Seawater challenge tests at days 5, 47 and 70 of freshwater rearing revealed no significant difference in plasma sodium concentration between groups. During the seawater phase of the experiment, the group receiving FinnStim had a significantly higher growth rate and a significantly lower concentration of plasma sodium at the end of the study. The improved growth performance in the group receiving FinnStim was due to a reduction in the number of salmon growing poorly.