The present study analyses the phytoplankton community (composition, diversity, abundance and dominance) associated with physical, chemical and climatic variables of floodplain lakes of river Kosi basin, North Bihar. Five groups of phytoplankton were recognized in this study comprised of 43 species. Out of these, Chlorophyceae was the dominant group with 20 species, followed by Bacillariophyceae (11 species) and Cyanophyceae (9 species). Euglenophyceae and Dinophyceae were the less dominant groups with 2 species and 1 species, respectively. During investigation, 33 species were recorded from Tarawe chaur and 26 species from the Gamharia chaur. The abundance and population density of the all groups of phytoplankton were recorded. Closterium lunula, Oedogonium globosum, Chlorella vulgaris, Cladophora sp. and Spirogyra sp. contributed bulk of the Chlorophyceae population. Bacillariophyceae was represented by Navicula trivialis, followed by Melosira granulate and Fragillaria sp. Cyanophyceae was dominated by Oscillatoria limosa, followed by Microcystis elongate and Anabaena constricta. The abundance of Melosira granulate, Navicula trivialis, Chlorella vulgaris, Closterium lunula, Desmidium sp., Anabaena constricta, Microcystis elongate and Oscillatoria limosa indicate that the Gamharia chaur is under the threat of eutrophication. Various ecological factors exerted influence on species composition and abundance of phytoplankton. Sorenson's index analyses for phytoplankton community show 57.63% similarity among two lakes and Cyanophyceae indicates greater similarity 71.43% in species composition.