The study of the most abundant components in freshwater plankton in the Gulf of Ob and Taz Estuary in the summer-autumn season has demonstrated that the abundance and biomass of bacteria are stable and typical for mesotrophic waters during active microalgae vegetation. The abundance of viral particles varies in the range which is reported for unproductive or medium-productive water bodies. The environmental factors affecting affect the development and patterns of bacterio- and virioplankton distribution are considered.