The localization and distribution of seven neuropeptides in the nervous system of the plerocercoid, adult and free proglottis stages of the tetraphyllidean tapeworm Trilocularia acanthiaevulgaris have been determined by an indirect immunofluorescence technique. Six of the peptides are vertebrate-derived, namely, pancreatic polypeptide (PP), peptide tyrosine tyrosine (PYY), vasoactive intestinal polypeptide (VIP), peptide histidine isoleucine (PHI), substance P (SP) and somatostatin (SRIF); the seventh is the invertebrate neuropeptide, FMR Famide. This is the first demonstration of VIP and SP immunoreactivity in a cestode parasite, and for SRIF this is its first description in any parasitic platyhelminth. Cell bodies and nerve fibres immunoreactive to PP, PYY, VIP, SP and FMRFamide are present throughout the CNS; the distributions of PHI and SRIF were more restricted. In the PNS, nerve fibres immunoreactive to PP occur in the bothridia, whilst in the free proglottis nerve fibres immunoreactive to PYY and VIP innervate the gonads; VIP-immunoreactive nerve elements also supply the reproductive ducts. Extra-neuronal sitings of peptide immunoreactivities were evident for PHI, in association with the excretory system, and for SRIF, in presumed tegumental cell bodies in the free proglottis. The results are discussed in relation to the possible roles of the peptides in the neurophysiology and developmental biology of the worm.