Ecdysis, the shedding of cuticle at the end of each life stage, is critical to the postembryonic development of insects. The endocrine regulation of ecdysis has been highlighted by the recent description of the epitracheal endocrine system in the tobacco hornworm Manduca sexta, which produces ecdysis-triggering hormone (Mas-ETH). This peptide hormone initiates pre-ecdysis and ecdysis through a direct action on the central nervous system. Here we show that ETH-immunoreactivity and ecdysis-triggering activity in epitracheal glands of the silkworm Bombyx mori are attributable to a 23 amino acid peptide, Bom-ETH. The complete amino acid sequence of Bom-ETH is SNEAFDEDVMGYVIKSNKNIPRM-NH2. Synthetic Bom-ETH was prepared and shown to be chemically and biologically identical to the native substance. Injection of Bom-ETH leads to pre-ecdysis and ecdysis in B. mori pharate larvae and pupae as well as comparable stages of M. sexta. Exposure of the isolated nervous system to Bom-ETH triggers pre-ecdysis and ecdysis burst patterns corresponding to the natural behavior. Bom-ETH belongs to an extended family of multifunctional neurohormones and hormones found in arthropods and molluscs.