The paper considers the ecological and taxonomic structure of the winter algocoenosis of the Chernaya, Kacha, and Belbek rivers within the territory of the city of Sevastopol. The results of studies were obtained on 30 chemical and physical parameters of surface water samples during the period of algological sampling. The collection of field material (phytoplankton, algae in fouling, on various substrates immersed in water) and its processing was carried out according to the methods generally accepted in algological practice. The diagnostic features, as well as the similarities and differences in the species diversity of algae, have been determined. The studies carried out show that the studied algocoenosis are characterized in winter period by a rich species composition, with a significant predominance of diatoms. The algae found in the studied rivers are indicators of water purity, and their occurrence activity and abundance can indicate that the content of organic matter in the waters of the Belbek and Kacha rivers is lower than in the Chernaya river. This is confirmed by the presence of algae – xenosaprobionts in the waters of the Belbek and Kacha rivers, which are indicators of clean, not polluted, with organic substances water.