The effect of thuringiensin on the ultrastructure of Tetranychus urticae Koch cuticle was studied using an transmission electron microscope (TEM). Thuringiensin was found to affect epicuticle synthesis in larvae, protonymphs and deutonymphs. Its disruptive effects on larval cuticle formation were observed from 12 h onwards, indicating that the effect of the exposure time of mites to thuringiensin is related to the stage of cuticle development. The results suggest that the effects of thuringiensin occur in a relatively short time.