The nervous system and chaetotaxy of a plagiorchiid cercaria,Haplometra cylindracea (Zeder), have been studied in order to recognize the relationships between the arrangement of sensillae on the body surface and the nerve trunks and commissures. The nervous system was revealed according to the methods of Koelle (1951) and Holt and Withers (1952) for localization of cholinesterase and nonspecific esterases. The silver impregnation method was used to visualize the sensillae. It has been found that the nervous system ofH. cylindracea cercaria differs from a hypothetical model given by Richard (1968, 1971) in the following characters: only two cephalic commissures occur instead of three, the median one is lacking, and the tail has only one longitudinal nerve trunk branching off from the last ventral commissure. The subtegumental nerve plexus has also been recognized. The chaetotaxy is described and the connections of particular groups of sensillae with corresponding nerve tracts are discussed.