Studies of the larval nervous system of two species of starfish were carried out using antisera to a recently isolated native echinoderm neuropeptide, GFNSALMFamide (S1), and to serotonin. S1-like immunoreactivity was found in the larvae of the asteroids Pisaster ochraceus and Asterias rubens (Echinodermata: Asteroidea), originating in the apical region and becoming concentrated as two groups of cells in the dorsal ciliary band, the preoral transverse and adoral ciliary bands in larvae up to the early brachiolarian stage (five weeks). The pattern of serotonin immunoreactivity, although appearing earlier in the apical nerve plexus, is very similar to that of the peptide, with paired groups of immuno- reactivity apparent in the dorsal ciliary band. This evidence, together with other recent studies, indicates that this neuropeptide is present in both the larval and adult nervous system, despite the complete reformation of the system at metamorphosis. The close localization of SI with serotonin may also suggest a possible function for the peptide in larval and adult nervous systems.