In order to investigate the possibility of fine-size cocoon production by the use of imidazole compound, Trifumin, the cocoon and cocoon filament qualities, dietary efficiency and fibroin component were examined. cocoon size, cocoon weight and cocoon shell weight of trimolter were smaller than those of tetramolter. Cocoon shell weight of trimolter was 58% and 26% of that of tetramolter in the 3rd instar treated larvae and in the 4th instar treated larvae, respectively. Amount of cocoon shell produced per 1g dry matter ingested was 94% and 85% of that of tetramolter in the 3rd instar treated larvae and in the 4th instar larvae, respectively. The effects of Trifumin on the reeling tests were different according to the stage of the treatment. When Trifumin was treated at the 3rd instar larvae, reelability of cocoon filament, amount of cocoon filament, size of cocoon filament, reelability percentage and raw silk percentage of cocoon were 100%, 60%, 60%, 97% and 90%, respectively. When Trifumin was treated at the 4th imstar larvae, most of reelability tests were inferior to those of the 3rd instar treated larvae, but reelavility percentage was increased 13%. The Trifumin treatment did not affect the electrophoretic patterns of cocoon fibroin.