Schistosoma sinensium Pao, 1959, was first isolated from an unidentified snail in Szechuan Province, China, and was described on the basis of adult worms and eggs recovered from experimentally infected mice. We discovered snails, Tricula bollingi Davis, and rodents, Rattus rattus (L.) and Crocidura attenuata Milne-Edwards, infected with S. sinensium near Fang in northwest Thailand. Adult worms were recovered from small veins immediately adjacent to the small intestine. Eggs first appeared in the feces of experimentally infected mice at 25 days postinfection. Cercariae first emerged from experimentally infected T. bollingi at 40 days postinfection. Shortly after emerging, cercariae were found at the water surface, their bodies flat against the meniscus and tails hanging free in the water column. Our morphometric results for adults and eggs from naturally infected hosts agree well with those presented in the original description. The occurrence of a lateral spine on the egg of this Asian schistosome has created some confusion as to the affinities of S. sinensium. A comparison of 8 character states, including snail hosts, cercarial behavior, geographical distribution, and morphology of life cycle stages, indicates that S. sinensium is more closely related to S. japonicum Katsurada than it is to S. mansoni Sambon.