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An isolate of Oscillatoria from 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 from Oscillaroriu. 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 in Anacystis nidulans.Algal bloom sequence in eutrophic freshwater lakes, is usually determined by several interacting factors, which include light and major nutrients (KLEIN and CRONQUIST 1967, SHAPIRO 1970). Consequently, the effects of extracellular metabolites (allelopathy, probiosis and antibiosis) have been suggested as playing major role in sequence determination within a phytoplankton population (KEATING 1977).A wide range of blue-green algae (cyanobacteria) exert inhibitory effects on growth of other algal species, and this interaction has often been attributed t o the extracellular metabolites, namely polysaccharides (and bacteriocin like proteins (FLORES and WOLK 1986).However, our knowledge t o the algicidal function of blue-green algae is, as yet, restricted to one species, Scytonenza lioffmunii (MASON et ul. 1982), whose functional metabolite has been well characterized (GLEASON et ul. 1986, GLEASON and PAULSON 1984, GLEASON and BAXA 1986, PIGNATELLO et ul. 1983.

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