Investigations of Diphyllobothrium infection of gulls and terns from the Beaver Island archipelago, Lake Michigan since 1936 disclose the following interrelations with lake herring, copepods, and young birds. Mayfly and caddisfly emergents during June and July coincide with the appearance of ciscoes in deep water near the surface and the deyelopment of young birds which are fed herring by the parents. The Diphyllobothrium is shown experimentally to mature in young laboratory hatched and reared gulls and Caspian terns in 3-1/2 to 4 weeks after which time they are usually shed. During this period the young are feathered and learn to fly. The tapeworm develops in birds in the rookeries similarly and is shed in waters about the islands in late July and August in great numbers. The circuitous water currents about the islands of northern Lake Michigan tend to confine the segments to that area. During August and September the herring retire to deep water and both old and young birds disperse in numbers to inland lakes.. The herring spaawn in the shoals about the islands in October and November and hatch in the spring. Experiments with Diphyllobothrium oblongatum eggs indicate they can withstand freezing for at least one month and at 20 C. for five years. Thus a continual source of infection is maintained for the first intermediate host, Diaptomus oregonensis which is one of the chief foods of the herring. The procercoid in the copepod becomes a plerocercoid in fish in from one to two months in cysts on stomach and mesenteries.