Fumigation toxicity of hydrogen cyanide (HCN) was compared to very potent natural and synthetic cyanohydrins against the house fly and the lesser grain borer. The concentration of cyanide ion was much higher in the house flies exposed to HCN fumigation than in the flies that were exposed to cyanohydrin fumigation or to topical application of cyanohydrins. The results indicate that the toxic action of cyanohydrins in insects is due to the combined effects of cyanide ion, the aldehyde or ketone produced by the decomposition of the cyanohydrin, and possibly the intact cyanohydrin molecule. © Presticide Science Society of Japan