The paper presents the first data on phytoplankton of the rivers Vitim, Mama, and Chuya located northeastward from Lake Baikal, Zabaikalie, within the upper part of the River Lena watershed, and description of their icy algae. Dynamics of main chemical components in these rivers during the study, March to September 2009, and structure of their planktonic algal flora were assessed. The phytoplankton have been quantitatively characterised and their seasonal dynamics has been described. It was shown that the river waters were low mineralised so that their ion sums did not exceed 138 mg⋅l^-1. Their maximal values were observed in the under-ice period when the rivers were fed mainly by the underground waters and minimal values in June and the beginning of July after snow-melting and when glaciers began to thaw. On the composition of the principal ions, the studied waters belong to the hydrocarbonate class, calcium group. On the species composition in plankton, there dominated diatoms, 36, 31, and 29 species correspondingly in Vitim, Chuya, and Mama. The benthic algae prevailed. Biomass values of microalgae were low but increased in summer, from 34 mg⋅m^-3 in Chuya to 230 mg⋅m^-3 in Vitim.