The research was carried out to determine the growth differences of rainbow trout in freshwater and seawater. Totally 480 individuals with initial mean weights of 88.08±0.91 g and 85.58±1.15 g were used in the experiment which was carried out in a freshwater facility of a commercial trout farm and the sea farm of Sinop Fisheries Faculty of the University of Ondokuz Mayıs. The experiment was conducted for 90 days. At the end of the experiment, rainbow trouts grown in freshwater and seawater, reached final average weights of 295.56±5.65 g and 388.41±8.12 g and the daily growth rates were found as 2.61% and 3.37%, respectively. In spite of the lower temperature in seawater (6.9°C in averange at the sea site while 8.4°C at the freshwater site), the growth of fish reared in seawater was about 46% superior (1.46 times) to that of their contemporaries at the freshwater site. Consequently, it has been concluded that breeding of rainbow trout in sea cages is more advantageous than in freshwater facilities.