Present investigation deals with systematic enumeration and importance of Genus Microcystis Lemmerm. collected from rivers. Eight species of Genus Microcystis Lemmerm. (Microcystis aeruginosa (Kütz.) Kütz., M. elabens (Bréb.) Kütz., M.flos-aquae (Wittr.) Kirchn., M. incerta (Lemmerm.) Lemmerm., M. protocystis Crow, M. pseudofilamentosa Crow, M. robusta (H.W.Clark) Nygaard and M. viridis (A.Braun) Lemmerm.) were recorded from Nanamau to Bithoor stretch of the Ganga. Microcystis aeruginosa (Kütz.) Kütz. was found in all seasons. Microcystis aeruginosa (Kütz.) Kütz. registered its presence in plankton counts, sand and water cultures. M. elabens (Bréb.) Kütz. and M. protocystis Crow were recorded rarely or once during the year. The investigation emphasizes the importance of various species of genus in water quality of river Ganga. Microcystis aeruginosa (Kütz.) Kütz. occurrence may produce certain types of hepatotoxins which may threat to cause acute to severe problems in animals and human beings.