We investigated the effects of PF4 on Ascaris suum somatic muscle cells using a 2 electrode current-clamp technique. PF4 is a FaRP (FMRFamide-related peptide), originally isolated from the free-living nematode Panagrellus redivivus. PF4 caused hyperpolarization and an increase in chloride ion conductance when it was applied to the muscle cells of the Ascaris body wall. The delay between the application of the peptide and the appearance of the response was measured and compared with that of gamma-amino butyric acid (GABA), a compound that directly gates ion channels, and with PF1, a FaRP that acts via an intracellular signal transduction mechanism. The PF4 and GABA delay times were not significantly different; they were 1·51±0·11 sec and 1·22±0·10 sec respectively. The delay following application of PF1, 3·75±0·51 sec, was significantly longer. The rapid response to PF4 is consistent with direct gating of a chloride ion channel, which has not been described elsewhere in the literature.