Deltamethrin is a pyrethroid insecticide that is considered extremely toxic to aquatic organisms. We evaluated the effect of subacute doses (0.033, 0.1, or 0.3 mg/kg) of deltamethrin on micronucleus frequency and oxidative stress markers in Rainbow Trout Oncorhynchus mykiss at 1, 2, 3, 4, and 8 d after intracoelomic injection. No genotoxic damage was recorded, as no specimen showed a micronucleus number above the physiological range. Deltamethrin exposure elicited a transient reduction in the levels of superoxide dismutase, catalase, glutathione peroxidase, and glutathione S-transferase and a persistent decrease in glutathione reductase activity. Overall, the lower antioxidant enzyme activity in the deltamethrin-treated fish was mainly dose-dependent. Received May 27, 2016 accepted June 18, 2017.