The total list of phytoplankton of pelagic and littoral zones of Onego Lake includes 775 species, varieties and forms of algae, which belong to 8 branches, 16 classes, 30 orders, 76 families and 155 genera. The diversity of Bacillariophyta, Chlorophyta, Cyanophyta and Chrysophyta is the most (about 93 % of species of the generalized list). Diatoms have the highest floristic proportion in the plankton flora. Phytoplankton of lake is noted for a big number of families and genera represented by one or few species. The plankton flora is mainly made up of cosmopolitan species with the essential share of boreal and arctalpine forms (53 % from the total list), species indifferent towards salinity (75 %) and pH of the water (51 %), oligosaprobic, oligo-β-mesosaprobic and β-mesosaprobic species (79 %).