Under laboratory conditions, the toxicity of acid water on several major freshwater fishes were studied. The results indicated that the H+ concentration was correlated with the mortality of embryo and fry of these fishes. The development of embryo was postponed at lower pH levels. In order to keep fertilizer eggs to hatch out normally, the pH level in water should be, at least, higher than 6.5. No significant difference in sensitivity was observed among the fishes tested. However, the sensitivity of pH of each fish species was distinct at different development stages. The sensitivity decreased as the fish developed. Pathological changes of lamellar epithelim were observed at pH 6.0. the toxicity of low pH and aluminium showed synergism and was affected significantly by hardness of water (or Ca-2+ concentration). Moreover, the softer the water quality was, the more harmful the toxicity of H+ and Al was.