PURPOSE: To obtain an insecticide free from the problem of secondary disaster caused by living spore, etc., by exterminating the living spores and living nurse cells existing in the culture liquid of Bacillus thuringiensis with iodine while keeping the insecticidal activity of crystal toxin.

CONSTITUTION: Bacillus thuringiensis is cultured in a natural medium rich in N source, C source, minerals and vitamins under sufficiently aerobic condition at 25-30°C for about 24hrs to obtain a culture liquid containing a crystalline proteinous toxin (crystal toxin), living spores, living nurse cells, etc. The water- soluble components are removed from the culture liquid to an extent not to lower the dispersibility of the above components and the pH is adjusted to 4-7 with sulfuric acid, etc., to sterilize the liquid. The living spores and the living nurse cells can be easily and completely exterminated by a combination of a sterilization treatment with 0.1N iodine solution and a physical sterilization treatment with heat, ultrasonic wave, radiation, etc., to obtain an insecticide having excellent insecticidal activity.