The shell-less, endoparasitic gastropod, Asterophila japonica Randall and Heath, 1912, was collected from two species of sea star, Leptychaster anomalus Fisher, 1906 and Ctenodiscus crispatus (Retzius, 1805) in Toyama Bay, Japan. All observed individuals were located on the aboral side of the host's disk (except one specimen parasitizing the arm) between the epidermis and the coelomic epithelium. More than one large individual frequently co-occur on a single host. The body plan of A. japonica is surprisingly modified from that of general gastropods; organs unrelated to digestion and reproduction are greatly reduced, simplified or completely lost. Dimorphism of body size is striking between males and females: males are much smaller than females and are attached to the surface of the pseudopallium of females. Females deposit and brood an egg mass(es) in the pseudopallial cavity until the eggs develop to veliger larvae. At the late developmental stage, brooded larvae reduce the velum and develop the foot for crawling, suggesting lecithotrophic development with or without a short planktonic stage. It is uncertain as to how the larvae can find and parasitize the next generation of the host. The systematic placement of Asterophila in the family Eulimidae is supported by three characters, viz. parasitism on echinoderms, smooth hydrophobic protoconch, and the enclosure of the visceral mass with the pseudopallium.