When cultured with an axenic planktonic Oscillatoria sp., growth of some cyanobacteria and a green alga was suppressed owing to release of an algicidal metabolite (antibiotic). Growth inhibition was also detected on exposure to a cell-free extract of Oscillatoria or to an ether extract. The ether extract abolished the photosystem (PS) II activity of Anacystis nidulans. Experimentation with a combination of photosynthetic electron acceptor-donor systems indicated that the antibiotic also inhibited PS II in isolated chloroplasts, primarily interacting at a site before P680 on the electron transport chain. The cyanobacterial product also suppressed growth of some natural isolates of cyanobacteria and green algae in controlled conditions and decreased the population density of a green alga (phytoplankton) in nature. Growth of higher plants was also affected, though only when the Oscillatoria extract was applied to leaves. Experimental mice were unaffected by the antibiotic. Upon thin layer chromatography of the ether extract, two fractions with algicidal activity fluoresced under ultraviolet light. Long-chain saturated fatty acids were the major components of the fractions. The suitability of the antibiotic in control of toxic algae is discussed.