AbstractAn isolate ofOscillatoriafrom local eutrophic lake exhibits strong growth inhibitory effects, when phototrophically co‐cultured with a green alga and other blue‐green algae, including its natural predecessor. This effect was not observed on selected heterotrophic‐organisms. The characteristic interaction was also demonstrated with culture filtrates and cell‐free extracts, implying this effect to be due to some chemical released fromOscillatlia.From the mid‐log growth phase of the culture, a nonproteinaceous, moderately heat stable, and ether soluble metabolite was extracted by ether extraction. The primary site of the growth inhibitory substance (algicide) action appears to be the photosynthetic oxygen evolution, tested inAnacystis nidutlans.The characteristic algicidal substance may have a role, in nature, in species