The effect of the hydraulic turnover rate on dominance of the cyanobacterium Microcystis was examined using field data from the summer of 2001 for the Minami-shio Reservoir, a shallow eutrophic regulating reservoir in Japan, where the water outflow and storage volumes are measured hourly. A Microcystis bloom occurred in late summer when the daily hydraulic turnover rate decreased to 15%. The hydraulic turnover rate provided an index of nutrient supply rate, because the nutrient load was input mostly in the inflowing water and because the water body was well mixed. Field measurement results indicate that the low hydraulic turnover rate led to the Microcystis bloom.