Abstract <p>The seasonal dynamics of zooplankton (Rotifera, Cladocera, and Copepoda) are studied in the Ob River in years of different water levels in the zone of influence of the Novosibirsk Hydroelectric Power Station and in the middle reaches of the Ob River. Zooplankton samples are collected and analyzed using standard methods. Research shows that the middle reaches of the Ob River are characterized by the formation of two summer peaks in zooplankton abundance and the presence of a clean water phase in late June and early July at the decline of the second flood wave and at the very beginning of the summer low-water period. In the section of the Ob River below the hydroelectric dam, the summer peak of zooplankton development is “cut off” due to the design features of water intake to turbines and the features of discharge regulation. In both sections of the river in the same season in different years, the number of zooplankton species, as well as the abundance and biomass of zooplankton, can vary significantly. Seasonal changes in the numerical indicators and species structure of the zooplankton community depend on the synergy of biotic and abiotic factors. During the flood period, the leading role in the formation of the zooplankton structure is played by the hydrological parameters of the watercourse, and during the summer low-water period, the water temperature. The spring and summer peaks in zooplankton abundance are mainly provided by rotifers, but the species composition varies significantly. The spring peak, as a rule, is ensured by the massive development of genera <i>Keratella</i> and <i>Synchaeta</i>, and in July–August, species of <i>Brachionus</i> and <i>Polyarthra</i> dominate in numbers. The change of taxa is determined by the biological characteristics of species. In both studied areas, regardless of the quantitative indicators of zooplankton, maximum values of the Shannon index were revealed in August.</p>