Microbiological assessment of the water quality in the mouth of the Kazanka River (Tatarstan Republic) was carried out in the summer of 2021. The total numbers of bacterioplankton, saprophytic, oligotrophic, denitrifying and nitrifying bacteria were determined. The water is classified as “slightly polluted” and “polluted”, i. e. quality class II-III, by all indicators. Bacterioplankton remained highly abundant throughout the study period. In June, oligotrophic bacteria outnumbered saprophytic ones, but in the end of the summer, saprophytic bacteria turned into the dominant group, pointing to water pollution and organic matter accumulation. Nitrifying bacteria were more abundant than denitrifying bacteria in the river mouth ecosystem, which indicates an active ammonium nitrogen mineralization process.