A Streptomyces strain, NT0401, was isolated from soil that selectively inhibited Microcystis strains but did not affect other microorganisms. Based on its morphology, physiology and 16S rDNA sequence, it was identified as Streptomyces grisovariabilis. The active substance produced by NT0401 was a water-soluble compound with a Mr < 1 kDa that was stable over a broad pH range and at 100A degrees C for 20 min. This organism should be a potential environment-friendly strain for control of Microcystis blooms.