Stephanostomum baccatum, an acanthocolpid trematode, has the following host succession in eastern Canadian waters: Buccinum undatum and Neptunea decemcostatum (Gastropoda), primary intermediate hosts; the six common species of pleuronenctids, second intermediate hosts; Hemitripterus americanus and Hippoglossus hippoglossus, common definitive hosts. Infection of flatfish is by penetration of the integument of the host by an atypical Ophthalmoxiphidiocercous cercaria.The infection of the winter flounder, Pseudopleuronectes americanus, as investigated in Passamaquoddy Bay, N.B., shows the following characteristics:1. The infection in inshore waters is greater near open water than on shoal grounds.2. For any given location deeper water samples show a higher incidence of infection than shoal samples.3. Larger fish have heavier infections than small ones.4. The growth of the flounder is not impeded by heavy cyst infections.5. No marked seasonal variation of infection can be demonstrated. Control is impractical.