Neuropeptide-FMRFamide-like immunoreactivity was characterized in the fruit fly, Drosophila melanogaster. In the adult central nervous system, a stereotypic pattern of immunoreactive cell bodies and immunoreactive nerve processes and varicosities was observed, indicating a neurochemical role for FMRFamide-like substance(s) in Drosophila. Localization of immunoreactivity in the central nervous system of early larval stage revealed that the majority of the prominent FMRFamide-like immunoreactive neurons were already differentiated. The FMRFamide-like immunoreactive neurons remain immunoreactive throughout postembryonic stage and persist in the adult central nervous system. In the larva, in addition to the central nervous system, FMRFamide-like immunoreactivity was localized in the fibers innervating the ring gland, in the ganglion innervating the gut and in the gastric caeca.