Daubaylia potomaca, originally described as a parasite of Helisoma trivolvis in the Potomac River, is now reported as a natural infection in H. trivolvis and H. campanulatum from a stream in the vicinity from a stream in the vicinity of the University of Michigan Biological Station, Cheboygan, Michigan. Transmission of D. potomaca from H. trivolvis to Australorbis glabratus was accomplished in the laboratory, and observations indicate that the life cycle may be completed in 15 days or less. Eggs, larvae, and adult nematodes of both sexes were found simultaneously in deep tissues. The known geographic distribution of the parasite and the pathology produced by it are described, and its possible use as a means of biological control for A. glabratus is noted.