The intertidal sand dollar Arachnoides placenta is parasitized by the eulimid Hypermastus placentae at Pallarenda Beach, Townsville, Queensland. A. placenta is the only echinoderm species in the region with which H. placentae associates. Sand dollars are distributed in an aggregated pattern within the littoral zone at Pallarenda Beach, and the distribution of parasites within the adult sand dollar population conforms closely to Poisson expectations. A. placenta adults in all size classes are equally susceptible to parasitization by H. placentae, whereas juveniles are not parasitized; host selection may be determined by a requirement for host gonadal tissue in the diet of the parasite. Infestation rate and number of parasites per host decrease as host density increases. Parasite body size is not related to host body size.