This experiment was conducted to understand the effects of organic selenium on rainbow trout, Oncorhynchus mykiss. Fish (33.47 +/- 0.15 g; n = 216) were randomly assigned to four treatment groups consisting of three replicates of 18 fish each in 100 x 100 x 125 cm cages. The fish were fed a basal diet supplemented with 2, 3, or 4 mg/kg organic Se (Sel-Plex (R)) supplementation for 8 weeks. Muscle of fish fed the unsupplemented control diet had a lower (p< 0.05) selenium content (4.78 +/- 0.12 mu g/g) than muscle of fish fed the supplemented feeds (6.45 +/- 0.18, 7.51 +/- 0.17, and 8.23 +/- 0.11 mu g/g for the 2, 3 and 4 mg/kg diets, respectively). The highest (p< 0.05) weight gain and specific growth rate were obtained in fish fed the 3 mg/kg diet. There were no significant differences (p> 0.05) in proximate composition except for protein content in fish fed the 3 mg/kg diet. Glutathione peroxidase (GSH-Px), catalase (CAT), and superoxide dismutase (SOD) activity was significantly higher (p< 0.05) and liver malondialdehyde (MDA) concentration significantly lower in fish fed the supplemented diets than in the control. Results show that 3 mg/kg Se supplementation is most effective for growth and hepatic antioxidant enzyme activity in rainbow trout.