It has recently been shown that niacin can be formned from tryptophan in mammals (1), but cannot be formed in the larval stage of insects, such as 'The authors are indebted to K. Nakamura, M. Takanami, and S. Higashi. This work was supported in part by a grant from the Science Research Fund No. 4009, Ministry of Education. Drosophila melanogaster (2), Tenebrio molitor, and Tenebrium confusum (3), which require both tryptophan and niacin in their larval growth. This paper reports an increase of the amount of niacin and niacinamide during the metamorphosis of the pupal stage in the silkworm Bombyx mori. We determined these two substances in various stages of the pupa, which live for a considerable period (7-12 days) without food. Female pupae of the Japanese and Chinese hybrid (J 115 x C 108) were used. The samples were kept at a constant temperature of 280 C. The determination of niacinalnide was made by Kato and Shimizu's method (4), and that of total niacin by the microbiologic assay method, using Lactobacillus arabinosus. The results of the experiments are given in Tables 1 and 2 and in Fig. 1. It was found that in the period from the second to the eighth day both the total niacin and the niacinamide reached a maximum on the eighth day, and that the amounts of these substances were twice as great as on the second day. They suddenly decreased on the ninth day. The formation of total niacin was found to be greater than that of niacinamide at each period. From the above results, we conclude that niacin and niacinamide are formed biosynthetically in silk-