The moulting process in insects is initiated and directed by the hormone 20-hydroxyecdysone1, which causes the secretion of a new cuticle and the internal changes that are necessary for the new stage. When the new cuticle is complete, the old one is then shed or ecdysed. In Lepidoptera a neurosecretory peptide, eclosion hormone (EH)2-4, triggers adult ecdysis by acting on the central nervous system to elicit the ecdysial motor programmes5 and also causes other physiological changes associated with ecdysis6. Although it was originally thought that EH was used only for the rather specialized adult ecdysis3, we report here that the hormone is used for all postembryonic ecdyses in the life history of moths, but its source varies with developmental stage. Moreover, EH may be common to a wide variety of insects. For the ecdyses of the adult4, pupa 7 and the 5th instar larva of the tobacco hornworm, Manduca sexta (P.F.C., unpublished results), EH appeared in the blood before the start of the behaviour. Also, injections of partially purified EH caused precocious ecdysis of the three stages with 50% responses occurring at 0.3, 0.02 and 0.01 EH units, respectively.