Rainbow trout weighing 0.9g on the average were fed purified diets containing different amounts of Mg for 6 weeks. The concentration of Mg in the rearing water was 3.1 ppm. Dietary Mg levels were found to affect not only the appetite, growth, and movement of the fish but also the contents of ash, Mg, and Ca of the whole body and of the vertebrae. The occurrence of vertebral curvature was also recognized in the fish which received the Mg-deficient diet. The values for Ca/Mg of the whole body and of the vertebrae changed according to the dietary Mg levels. Histological changes were recognized in the muscle, pyrolic caeca, and gill filaments from the fish fed the Mg-deficient diet. The requirement of rainbow trout for dietary Mg was estimated to be 0.06-0.07% of a dry diet or 12-16mg per 1kg body weight per day under the present experimental conditions.