Data on the status of summer phytoplankton of the Usa river and its tributaries are given. 184 taxa of algae with a rank lower than the genus from 7 divisions were recorded in algoflora, with Bacillariophyta (43-57% of the composition) and Chlorophyta (23-47%) predominating in the rivers. The abundance range in algocenoses was 0.02 - 5.2 mln cells/L, that of biomass was 0.01 - 1.8 mg/L; the content of chlorophyll-a was 0.01 - 3.1 µg/L.