Cadmium sulfate and zinc sulate salts in graded concentration were fed to silkworm larvae using an artificial diet. Tests were done at three periods, from hatching to pupation, from the beginning of the 3rd instar to pupation, and in the last (5th) instar period. The results obtained are as follows: when the concentration of Cd (as Cd element of CdSO/sub 4/) in diet was over 50 ppm in the 1st instar test and over 200 ppm in the 3rd and the 5th instar tests, all insects were dead before pupation. When the concentration of Cd was over 10 ppm in all tests the cocoon layer became lighter than controls. We conclude that the safety level of Cd concentration in diet is 5 ppm. When the concentration of Zn (as Zn element of ZnSO/sub 4/) in diet was over 200 ppm in the 1st and the 3rd instar tests and over 400 ppm in the last instar test, the cocoon layer become lighter. We conclude that safety level of Zn concentration in diet is 100 ppm.