Among the environmental problems in Nigeria is the lack of proper management of industrial waste-waters, which is capable of causing toxicity to aquatic organisms via leachate or direct discharge to the surrounding aquatic environment. In this study, characterization of an oil mill company wastewater revealed that concentration levels for some of the parameters (Total hardness -17726.15 ± 18.14 mg/l.CaCO3, Electrical conductivity - 756.17 ± 21.43mS/cm, BOD - 87.39 ± 0.47mg/l, PO4-P -2535.87 ± 30.10 mg/l, NO3-N -102.67 ± 14.22 mg/l, and Sulphate - 16500.21 ± 65.47 mg/l) have potential toxic effects. The acute toxicity of the wastewater to fingerlings of Clarias gariepinus was determined in 96-hour static renewal bioassays. Symptoms of toxicity observed in the fish include various behavioral responses like rapid and erratic swimming, uncoordinated movement, vertical darting, with occasional agitated movement, swimming alternately on the lateral and ventral sides and rapid opercula movement. Prior to mortality recorded at some concentrations, there was negative thigmotropism with prolonged gaping of jaws. These observations as well as mortality records were dose – dependent. The toxic action of the oil mill company wastewater appeared to be a joint consequence of precipitation of mucus on the gills and stress. These changes may have lead to an overall reduction in the efficiency of the gill filament to aid diffusion of oxygen across the gill filament, resulting in death. The traces of blood observed around the gill covering, tips and base of tail fins and barbels on dead fishes and the reduced PVC in some concentration tanks suggest that the fish might have suffered from gill haemorrage. In the LC50 study, no death was recorded in the control. The mortality in the various test concentrations tanks varied between10% to 80%. The 96 hr LC50 obtained using the probit method was 55.3623ml/ with 95% confidence lower and upper limits of 44.9268ml/l and 67.3248ml/l respectively. The threshold for the oil mill wastewater is 98.4130ml/l.