Feeding trials were conducted in two consecutive years to study the effect of oxidized fish oil on the stability of ascorbic acid in practical trout diets. The vitamin C nutrition of fish fed these diets was also investigated. Fish oils were oxidized by bubbling air through them for 0, 100, 265, 1348 and 1540 hr and 7.5% of these oils were then added to basal diets containing 400 mg ascorbic acid per kg diet. Total dietary ascorbate in the first trial was significantly (P less than 0.05) reduced by 24 wk storage at room temperature but was not significantly (P greater than 0.05) affected by the addition of moderately oxidized oil (peroxide value [PV] = 26 meq/kg oil). In the second trial, dietary ascorbate levels were significantly (P less than 0.05) reduced by the addition of highly oxidized (PV = 120 meq/kg oil) and extremely oxidized (PV = 314 meg/kg oil) fish oil as well as by 24 wk storage. Fish fed the control and experimental diets for 24 wek showed no significant (P greater than 0.05) differences in live weight gain, feed/gain ratio, mortality, total liver ascorbate concentration, red blood cell count, hemoglobin, or hematocrit in the 1978 trial. However, mortality, total liver ascorbate concentration, red blood cell count, hemoglobin and hematocrit were significantly (P less than 0.05) affected in fish fed diets containing extremely oxidized oil but no DL-alpha-tocopheryl acetate in the 1979 trial. The vitamin C status of rainbow trout fed diets containing oxidized oil is discussed.