1. Using immunocytochemical techniques and confocal scanning laser microscopy, the proteocephalidean cestode, Proteocephalus pollanicola from Lough Neagh pollan (Coregomus autumnalis) was examined for the presence of the native platyhelminth neuropeptide, neuropeptide F (NPF). 2. An antiserum specific for whole-molecule NPF (1-39) (Moniezia expansa) did not immunostain nerve processes in P. pollanicola. A C-terminally-directed NPF (30-39) (M. expansa) antiserum immunostained nerve fibres and cell bodies of both the central and peripheral nervous systems, including innervation associated with the female reproductive system. 3. The pattern of immunoreactivity was identical to that obtained using antisera to the C-terminal region of mammalian NPY-superfamily peptides and the invertebrate neuropeptide, FMRFamide. 4. Under radioimmunoassay conditions, only the C-terminally-directed NPF antiserum cross-reacted with the P. pollanicola peptide and detected 58.74 ng/g wet weight of NPF-1 R in extracts of the worm. 5. Chromatographic characterisation of P. pollanicola NPF-immunoreactivity indicated an apparent molecular weight of 4400-4700 Da, similar to that of NPF (M. expansa). 6. Further analytical HPLC characterisation identified two molecular forms of P. pallanicoda NPF-immunoreactivity, both of which had different retention times from those of NPF (M. expansa) and NPF (Artioposthia triangulata). 7. These data suggest that P. pollanicola possesses a neuropeptide which is homologous in its C-terminal region to NPF (M. expansa) but differs in its mid- to N-terminal region.