AbstractThe structure and distribution of zooplankton of the Tsimlyansk Reservoir were studied in August–September 2018. It was found that the zooplankton abundance was halved in the upstream part of the reservoir (2.4 ± 0.8 g/m3) as compared to the zone near the dam of the Tsimlyansk hydroelectric power plant (1.2 ± 0.6 g/m3). Copepoda constituted 70% of the biomass. The Ponto-Caspian Heterocope caspia and the invasive species Calanipeda aquaedulcis, Thermocyclops taihokuensis, and Acanthocyclops americanus were abundant copepods. The dynamics of zooplankton abundance in the reservoir is analyzed for more than 65 years since its creation (1952–2018) based on the published data. A more than twofold (up to 2.1 ± 0.5 g/m3) increase in the community biomass was revealed in 2015–2018 as compared with the 40-year period up to 2013 (0.8 ± 0.1 g/m3). The trends in the dynamics of the zooplankton abundance in the Tsimlyansk Reservoir, including the importance of the Ponto-Caspian crustaceans and invaders from other regions, are discussed.