Modern phylogenetic schemes propose a free-living turbellarian-like organism as the ancestor of all Bilateria, including chordates. While neuropeptides are now known to be of ancient origin within the nervous systems of invertebrates, the neuropeptides of turbellarians remain virtually unexplored. Here we report the primary structure of an FMRFamide-related pentapeptide, Gly-Tyr-Ile-Arg-Phe amide (GYIRFamide), isolated from an acidified ethanolic extract of the aquatic triclad turbellarian, Dugesia tigrina. Previously, we reported the primary structure, RYIRFamide, from the New Zealand terrestrial turbellarian, Artioposthia triangulata, widely regarded as a most atypical species. However, the present data implies a high degree of primary structural conservation between the FMRFamide-related peptides of both species despite the zoogeographical isolation of the New Zealand form for several hundred million years. In addition, while authentic FMRFamide itself has been identified in many molluscs and a few annelids, the current data would suggest that turbellarian analogues are typically pentapeptides with an X-Y-I-R-Famide motif. A protein/peptide database search revealed that the sequence GYIRF is contained within the primary structure of several bacterial Ni/Fe hydrogenase B-type cytochrome subunits and several proliferating cell nuclear antigens (cyclins) from plants.