The insecticidal action of phytoecdysones, i.e. ponasterone A, ecdysterone, inokosterone and cyasterone was investigated for various insects. The high insecticidal action was found when phytoecdysones were acted on the body surface of Chilo suppressalis larvae, Pluttella xylostella larvae and Culex pipiens molestus larvae. However, Musca domestica larvae and Pieris rapae crucivora were tolerant against the substances. The insecticidal acitivity for the rice stem borer larvae varied with the developmental stage. That is, the nondiapausing final instar larvae were more sensitive against the phytoecdysone than the 3rd instar and the diapausing larvae.