Cercaria batillariae is a digenean trematode parasite of Batillaria attramentaria, a Northwest Pacific snail (Shimura and Ito 1980), introduced to the West Coast of North America (Wasson et al. 2001). Digenean trematodes are platyhelminths, with complex life cycles and multiple hosts. Cercaria batillariae was discovered infecting the Japanese snail Batillaria attramentaria (=B. cumingi, Japanese False Cerith). The egg, deposited in bird feces, hatches into a microscopic tadpole-like larva called a cercaria. The cercariae infect various predators, such as snails, crabs, or fish, as metacercaria, which form cysts. The metacercaria consume the snail's gonadal tissue. reducing the snail's reproduction, resulting in 'parasitic castration'. When these prey are eaten by a bird, the trematodes excyst and mate. The eggs are deposited in the sediment with feces, where they may be ingested by a snail. In spite of the reproductive losses from the parasite, Batillaria attramentaria, in California, out-competes the native snail Cerithidea califonica (California Horn Snail), because Batillaria has only one major parasite, white Cerithidea has 17 native parasites,