Over the last two centuries, anthropogenic activities have increased the nitrogen amount in aquatic ecosystems, which has resulted in increased occurrences of blooms of cyanobacteria. This study investigated the effects of nitrite and the cyanobacterium Microcystis aeruginosa on population growth in the rotifer Brachionus calyciflorus. The rotifer was treated for 12 days with nitrite alone (medium containing 0, 3, 6, 10 mg NO (2)(-) -N L(-1)), M. aeruginosa alone (medium containing 0 mg NO(2)(-)-N L(-1) + 5.0 ×ばつ 10(5) cell ml(-1) M. aeruginosa precultured at 0, 3, 6, 10 mg NO(2)(-)-N L(-1)), and nitrite in combination with M. aeruginosa (medium containing 3, 10 mg NO(2)(-)-N L(-1) + 5.0 ×ばつ 10(5) cell ml(-1) M. aeruginosa precultured at corresponding nitrite concentrations). We observed that a nitrite concentration of 10 mg NO(2)(-)-N L(-1) markedly inhibited the growth of B. calyciflorus; however, rotifer growth declined slightly in the presence of M. aeruginosa precultured at 6 mg NO(2)(-)-N L(-1). Furthermore, reduced population growth of B. calyciflorus was observed when it was treated with both nitrite and M. aeruginosa compared to nitrite alone or M. aeruginosa alone. These results suggested that a high tolerance of B. calyciflorus to nitrite levels may be attributed to the absence of specific respiratory structures and pigments; and that the increased toxicity of nitrite in combination with M. aeruginosa may have been due to increased production of microcystin. It is also possible that nitrite and microcystin could act in a synergistic way in causing toxicity.