The flimsy cocoon mutant of the silkworm, Bombyx mori, is characterized by its thin-layered cocoon-shell. This character is controlled by a recessive gene (referred to as flc) located at 49.0 position on the 3rd chromosome. The investigation of the pattern of action of the flc gene was mainly carried out biochemically and histochemically. Materials used are larvae and cocoons homozygous for flc of the strain b-60 from Institute of Silkworm Genetics, Kyushu University. As a control, the normal silkworms heterozygous for flc are also used. Results obtained are as follows: 1. Flimsy cocoon mutant larva grew as good as the normal and the body weight of flc larva was also similar to that of the normal. 2. The number of cells of the silkgland in flc larva was the same as that of the normal. 3. On the 1st day of the 5th instar the weight and the shape of the silkgland of flc were the same as those of the normal. But owing to the decrease of gelatinous silk protein secreted into the lumen, the weight of the middle silkgland of flc became much lighter than that of the normal, and the shape of the silkgland of flc deflated after the 3rd day of the 5th instar. 4. The cocoon-shell weight of flc was about one third of the normal and 90% of this decrease was caused by the less amount of fibroin. 5. The electrophoretic analysis of both cocoon-shell protein and gelatinous silk protein as well as the analysis of amino acids of cocoon fibroin were performed. As a result, it is indicated that the flc larva synthesizes the same protein qualitatively as that of the normal. 6. The free amino acid composition in hemolymph of the flc larva was similar to that of the normal to the 3rd day of the 5th instar. Afterwards the 5th day the concentration of specific amino acids, such as glycine and serine, became larger, and the total concentration of amino acids, except for sulfide containing amino acids, also increased as compared with those of the normal. 7. The incorporation of ^<14>C-amino acids into posterior part of the silkgland of flc was less than that of the normal, especially reducing by half on the 5th day of the 5th instar. These results show that the accumulation of a great amount of free amino acids in body fluid of flc is caused by the decrease of incorporation of amino acids into the posterior silkgland. 8. A peak of incorporation of ^<14>C-amino acids into protein of the normal posterior silkgland both in vivo and in vitro was seen on the 4th day of the 5th instar, whereas no peak was recognized in the posterior silkgland of flc and also the quantity of incorporation was distinctly reduced. 9. Both the DNA concentration and the incorporation of ^<3>H-thymidine into DNA of the posterior silkgland of flc were equivalent to those of the normal. Therefore, it is thought that DNA synthesis in the silkgland of flc is almost identical with that of the normal. 10. The RNA concentration of the posterior silkgland of flc was equal to that of the normal to the 3rd day of the 5th instar, but became 75% of the normal during the 4th to the 5th day. On the other hand the incorporation of ^<3>H-uridine into RNA of the posterior silkgland of flc was also equal to that of the normal through the 5th instar. These results suggest that RNA molecules of the posterior silkgland cells of flc are possibly degraded during the 4th to 5th day. 11. The cross section of the posterior silkgland of flc was observed with a light microscope. Afterwards the 3rd or 4th day of the 5th instar, nuclei of the posterior silkgland of fle stop developing and cell organella of the silkgland cell of flc degenerate and fibrous granules are accumulated in the cytoplasm of the fie cell, but not in the normal. There are many vacuoles or vesicles in the cytoplasm of the posterior silkgland cells of matured flc larva, but not in the normal one. From all these observations it is thought that on and after the 4th day of the 5th instar, the posterior silkgland of flc is going to break down. 12. Consequently, the fic mutant character due to a thin-layered cocoon-shell may be considered as the effect of the fic gene on the fibroin synthesis, not on the secreting ability itself, in the posterior silkgland cells.