The geographical distribution of Schistosomatium douthitti seems to coincide with the distribution of its snail hosts Lymnaea stagnalis and its varieties and Stagnicola palustris and its varieties. Another lymnaeid snail, Pseudosuccinea columella, is demonstrated to be a new susceptible host, and the nutria (Myocaster coypus) is found to be a good host and may be important in the dissemination if it migrates into new areas where the parasite does not occur at present. New information, relevant to transmission, is given on the biology of the unhatched and hatched miracidium, the development of the intramolluscan stages, the cercaria and its penetration of, and development in, the mammalian host.