For the first time the agent of diphyllobothriosis Lake Baikal is the cestoda D. dendriticum establishted by T.P. Chizhova and P.G. Gofman-Kadoshnikov (1962). D. dendriticum reaches maturity in human intestine, but it is considered, that the relationships between the parasite and human are facultative property (Klebonovski 1985). The results of the investigation of the diet of sea-gull nestlings (control-uninfected and experimental-infected birds with cestoda groups) and stress influence (starvation) on the mass loss in experimentally and naturally infected ones with different duration conditions adaptation of experiments are discussed in this article.