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We demonstrated the existence of the diversity of virus-like agents killing Microcystis aeruginosa in a shallow hyper-eutrophic pond in Japan, but without specific virus isolation, once in October and twice in December 2001. The pond water was treated by chloroform, filtered throughout GF-75 filter (0.3 µm) and then incubated with axenic M. aeruginosa for 7 days. The cell density of M. aeruginosa decreased 10-fold in the course of the incubation. Results suggested that the active agent for cell death was virus-like, based upon its size fraction (from 30 kDa to 0.3 µm), sensitivity on heat-treatment, and evidence of protein protection of DNA during our extraction procedure. From our results of pulsed field gel electrophoresis (PFGE) analysis, two or four different-sized DNA bands appeared from the culture lysate in each experiment. In total, 10 different-sized bands (10–90 kb) were observed from the three sample …