The phytoplankton from the pelagic and littoral areas of Lake Onegо comprises 426 species, varieties and forms of diatoms (55 % of the checklist) belonging to 2 classes (Centrophyceae and Pennatophyceae), 7 orders, 18 families and 44 genera. Out of 399 salinity indicator species and 392 pH-indicators, indifferent species comprise 300 (75 %) and 203 (52 %) species correspondingly. Out of 245 indicator species of saprobity 27 % are oligosaprobic, 22 % - oligo-p-mezosaprobic and 26 % - р-mezosaprobic. The largest share in the total abundance and biomass of the phytoplankton in the lake throughout the year is formed by the diatoms.