The aim of this study was to evaluate the effects of dietary diphenyl diselenide [(PhSe)2] at different concentrations (1.5, 3.0, and 5.0mg/kg) on growth, oxidative damage and antioxidant parameters in silver catfish after 30 and 60days. Fish fed with 5.0mg/kg of (PhSe)2 experienced a significant decrease in weight, length, and condition factor after 30days and these parameters increased after 60days. Thiobarbituric acid reactive substances (TBARS) and protein carbonyl (PC) decreased in the liver of silver catfish supplemented with (PhSe)2 after 30days at all concentrations, while after 60days these parameters decreased in liver, gills, brain, and muscle. Supplementation with (PhSe)2 induced a decrease in catalase (CAT) activity from liver only after 60days of feeding. Superoxide dismutase (SOD) decreased at 5.0mg/kg after 30 and 60days and glutathione peroxidase (GPx) was enhanced at 1.5 and 3.0mg/kg after 30 and 60days. Silver catfish supplemented for 30days showed a significant increase in liver glutathione S-transferase (GST) at 3.0mg/kg, while after 60days GST activity increased in liver at 1.5, 3.0, and 5.0mg/kg and in gills at 3.0 and 5.0mg/kg of (PhSe)2. After 30days, non-protein thiols (NPSH) did not change, while after 60days NPSH increased in liver, gills, brain, and muscle. In addition, ascorbic acid (AA) levels after 30days increased in liver at three concentrations and in gills and muscle at 1.5mg/kg, while after 60days, AA increased at all concentrations in all and tissues tested. Thus, diet supplemented with (PhSe)2 for 60days could be more effective for silver catfish. Although the concentration of 5.0mg/kg showed decreased growth parameters, concentrations of 1.5 and 3.0mg/kg, in general, decreased oxidative damage and increased antioxidant defenses.