In the Rangeley Lakes region of western Maine, where Diphyllobothrium sebago (Ward) is enzootic, the spawning of the smelt (Osmerus mordax) in late April and early May and the ensuing die-off of the spent fish coincide with the return and reproductive activities of the gulls (Larus argentatus). The gulls feed on the dead plerocercoid-infected smelt, which results in a high infection during the late spring and early summer. Thus, from early July onward, at least until the fall turnover in the lake, a constant source of coracidia is available for the copepod, the first intermediary, which constitutes the chief food of the smelt. The procercoid in the copepod becomes a plerocercoid in the smelt, which serves as a source of infection for the gulls, during either the spring-summer of the next year or of the subsequent year. Epizootiologically, due to the prolonged preservation of the plerocercoids in the fish, the smelt is the natural reservoir. Despite the common occurrence of the plerocercoids in the salmon (Salmo salar), its role as an intermediary is negligible.