Killifish were kept in distilled water or in river water, containing 10^-5 M-CdSO₄. The LT₅₀ of the fish in the distilled water containing cadmium (Cd) was 31 hrs, whereas that in the river water containing Cd was 327 hrs. The Cd contents of the fish increased with the increase of keeping day, and the contents and the uptake rate of Cd in the river water was lower than those in the distilled water. From the observations it seems that river water had a mitigative effect against Cd toxicity. The contents of zinc, iron, calcium and copper in the fish exposed to Cd were examined, and a significant decrease of zinc was observed both in the distilled water and in the river water. The high rate of decrease of calcium and iron in the dead fish and the relatively low rate of those in the alive were observed in the distilled water. The decrease of calcium in the alive kept in the river water was also significant, however, that of iron was not. The contents of copper of the fish was not changed significantly in both experiments. Hence, it is supposed that the toxicity of Cd in the fish is caused by the increase of Cd and by the decrease of zinc, calcium and iron incident to uptake of Cd.