The monograph presents the results of many years of research (1966-1993. ) zooplankton of Lake Onega, carried out as part of complex expeditions of the Institute of Water Problems of the North under the program "Investigation of patterns of formation and functioning of the ecosystem of Lake Onega under the influence of anthropogenic factors" . It provides a list of the species composition of the community (simple, collapsible, crustacean), analyzes its structure, patterns of spatial distribution and quantitative development in the annual cycle. Attention is paid to the issues of productivity and functional importance of the main groups of zooplankton and the whole community in the lake ecosystem. The role of small-sized fraction in zooplankton community . The book is designed for hydrobiologists, ecologists, conservationists.