A proteinaceous toxin has been extracted from sporulated cultures of Bacillus sotto Ishiwata. The toxin is similar in amino acid composition and biological activity to the crystalline inclusions produced by this microorganism and it appears that the toxic protein is associated with the crystals. The toxin is stable, and causes paralysis and death in the larvae of Bombyx mori L. (the silkworm) and other lepidopterous insects. The symptoms caused by the toxin are identical with those caused by ingestion of the whole microorganism.