A cyanobacterial bloom in the lower part of the Nakdong River was investigated during the dry summer of 1994. High levels of phytoplankton biomass, mainly Microcystis aeruginosa, in the surface waters (chl. α 193 ± 436 µg L-1, mean ± s.d.; >105 cells mL-1 , n = 15) were maintained for three months from mid July to mid October. After the last major rainfall in mid June, water temperature increased sharply within three weeks (18 June, 24°C; 9 July, 33°C). The highest cell density (5 × 106 cells mL-1) and highest concentration of chl. α (>500 µg L-1) in the surface water were recorded in the early phase of the bloom (21–26 July) as the drought persisted. Concentrations of dissolved inorganic nitrogen (DIN) and total phosphorus (TP) during the bloom were high (DIN 2.5 ± 0.9 mg L-1 ; TP 155 ± 98 mg L-1 ; n = 23). pH was low (~7) until the initial stage but was high (pH >9) as the bloom formed. Elevated water temperature (>30°C) along with low discharge and high irradiance were major factors contributing to the Microcystis spp. bloom in this river–reservoir system.