In an effort to develop a microbial in secticide, B. thdringiensis var. thuringiensis was cultured in the medium composed of cocoon-cooked water from a filature. The results obtained are summarized as follows : ① Bacillus thuringiensis is a bacterium producing a δ-endotoxin especially toxic to lepidopterous insects and a thermostable exotoxin harmful to dipterous insects. ② With a view to utilizing the cocoon-cooked water discarded from the filature, as a nutrient source in the B. thuingiensis culture, it was analyzed to contain large amounts of various minerals and protein (7.5 ㎎/㎖) believed to be extracted from the pupae. ③ A large amount of the δ-endotoxin can be obtained most cheeply by using cocooncooked water instead of distilled water in preparing GYS and citrate salts media. ④ The largest amount of a mixture of the vegetative cells, spores, and crystals was obtained by addition of 8 gr/ℓ of glucose to the GYS medium. ⑤ The growth of the bacterium was far better, when leucine, isoleucine, and valine were added all together to the citrate salts medium to the concentration of 1.25×10^(-3) M. ⑥ The best growth was observed by addition of Na-glutamate to the citrate salts medium to the concentration of 2. 5×10^(-3)M. ⑦ The optimal culture time ranged from 9 to 15 days. ⑧ The highest mortality was shown in Pieris rapae Linne with a pH of the total body extract of 8.4, whereas Dendrolimus spectabihs Butler and Bombyx mori Linne with lower pH`s were less susceptible to the δ-endatoxin. ⑨ The presence of the thermo stable exotoxin was confirmed by the fact that the supernatant of the culture was very toxic to the Drosophila melanogaster tested.