Healthy rainbow trout having mean length 13.86 ± 0.97 cm and mean weight 25.26 ± 0.52 g, were obtained from local fish farming, Elazig, Turkey. They were brought to fish diseases laboratory in Fisheries Faculty and acclimatized to the laboratory conditions for 15 days under natural photoperiod and ambient temperature. During this period fish were fed ad libitum with pellet feedstuff twice a day. In this study, 45 rainbow trout were used and 15 fish for each experimental group placed in three different 500 L aerated tanks, one of them used for control group and two for the experimental group. During 5 days, the fish were daily exposed to 1/15,000 (66.67 mg/L) and 1/150,000 (6.67 mg/L) concentrations of malachite green for 30 s and 60 min, respectively. Malachite green was supplied from a chemist. Two different concentrations of malachite green with three replicates were used in the test series. Control units with three replicates were also prepared.