The routine or short-term bioassay is commonly used for evaluating acute toxicity of industrial wastes to fish. The method does not measure chronic or cumulative toxicity resulting from exposure to comparatively low concentrations over a long period. Knowledge of chronic toxication is necessary in determining the concentration of a pollutant which will be safe for aquatic life. The present study was made to determine the effect of oil refinery effluents on fish when applied for extended periods under conditions of continuous renewal. Results from long-term bioassays were compared with those obtained from routine bioassays. Chemical analyses were made on effluents tested and factors affecting toxicity were considered.