Rainbow trout Oncorhynchus mykiss with an initial mean weight of 174 g (1+ year old) were fed on 3 different commercial diets (1 extruded pellet and 2 steam pellet) in the same environmental conditions until their first spawning. Changes in growth performance (weight gain, condition factor, specific growth rate and feed conversion rate), gonadosomatic index (GSI), hepatosomatic index (HSI) and body composition (protein, lipid, ash and dry matter) were investigated. The mean weight gain and condition factor for fish fed the extruded diet was significantly greater than for those fed the steam pellet diets (P < 0.05). There were no significant differences in specific growth rate, mean length increments and feed conversion rate among the groups, although the feed conversion rate for the extruded diet was slightly lower than the others. The HSI of the group fed an extruded diet was significantly greater than those of the others ( P < 0.05). The GSI values of female and male fish at the end of the study (October) did not differ significantly among the groups. Similarly, diet had no significant effects on the muscle composition of the trial fish, except that the muscle lipid of the group fed the extruded diet was significantly higher than those of the other groups (P < 0.05).