Newly ecdysed fourth instar larvae of the cotton leafworm, Spodoptera littoralis (Boisd.) were treated with LC₅₀ of ecdysone agonist methoxyfenozide, to provide better insights into physiological symptoms and aspects induced by it as a mimic to the 20-hydroxyecdysone (20-E) action. Larvae ingest methoxyfenozide died within 2-3 days, and being trapped within their excuvae. They stopped feeding shortly before ecdysis. Treatment neither accelerated ecdysis nor ceased feeding, as soon as the larvae ingest such 20-E mimic. The continuous binding of the compound to the ecdysteroid receptors of S. littoralis larvae increased the titre of enzymes regulated by 20-E during the experimental time (3 days). Chitinase and phenoloxidase were activated after 6 and 12 hr from methoxyfenozide administration, respectively. The compound had no effect on chitin production, but endocuticle chitin degradation during moulting might be depressed as illustrated by the presence of higher amount of N-acetylglucosamine in control larvae than methoxyfenozide treated. It is suggested that methoxyfenozide might induces a precocious moult by initiating moulting cycle, but its main toxic effect is due to the fact that its level remains high during moulting and don't decrease for allowing other hormones necessary for successful moulting to be released.