Toxicity of inorganic mercury to the freshwater fishTilapia mossambica was demonstrated by static bioassays. Statistical analysis was used for calculating 48 hr median tolerance limit, 95% confidence limits and the slope function (S). No mortality was observed within 48 hr in mercuric chloride concentration of 0.7 mgl−1 although the swimming activity of the fish decreased and the rate of opercular movement increased significantly. Concentrations of 0.9 mgl−1 and higher proved lethal. Symptoms of mercury poisoning and safe concentrations of mercury have been discussed.