Cyanobacteria are widely distributed in all natural ecosystems. These organisms form blooms under condition of nutrient over-enrichment, high temperature and low velocity. These blooms are harmful to the environment, animals and human health. The present study involves bloom forming cyanobacterial flora from Mahanadi River Near Hirakud Reservoir located in Sambalpur district, Odisha, India and from lower part of the hirakud dam. From the water sample, 37 species of cyanobacteria belonging 17 genera were recorded. Of these species, 9 were unicellular, 9 non-heterocystous filamentous and 19 heterocystous filamentous forms. Genus Calothrix with 7 species, Nostoc with 5 species, Cylindrospermum and Aphanocapsa with 4 species each were recorded. According to literature, 7 genera of them are potential toxin producer (Anabaena, Calothrix, Cylindrospermum, Hapalosiphon, Microcystis, Nostoc and Phormidium).
