Two planktonic cyanobacteria, Anabaena sp. N1444 and Anabaena sp. PC-I, and a green eukaryotic alga, Scenedesmus sp,, produced extracellular flocculants. The flocculant of Anabaena PC-I, when purified, was found to be a macromolecular polysaccharide consisting of neutral sugars, uronic acids, and proteins, but not keto acids, hexosamines nor fatty acids. The flocculant bound a cationic dye, Alcian Blue, indicating it to be polyanionic. The flocculating activity was high under acidic conditions, slightly enhanced by the addition of salts and metals, and increased to about 40% upon heating at 100 degrees C for 7 min. The flocculant could flocculated various inorganic and organic compounds in solution.