The pattern of serotonin-like immunoreactivity in the central nervous system of individuals of the amphipod Gammarus lacustris harboring polymorphid cystacanths was compared to the pattern seen in individuals not infected with acanthocephalans. Ventral nerve cords from both parasitized and nonparasitized amphipods showed the same bilateral pair of immunoreactive cell bodies in the third thoracic ganglion. Significant differences were noted in the fine structure of these cell bodies, with nerve cords from Polymorphus paradoxus-parasitized amphipods showing a greater number of bright spots or localized points of storage of serotonin. The results of this study indicate that infection of G. lacustris by cystacanths of P. paradoxus, but not P. marilis, is correlated with changes in the anatomy of the serotonergic neurons of the amphipod's central nervous system.