The growth inhibition of ethyl-2-methylacetoacetate (EMA) isolated from common reed (Phragmites australis Trin. or Phragmites communis Trin.) on the growth of Microcystis aeruginosa PCC7806 was investigated and the intracellular and extracellular concentration of cyanotoxin (MC-LR) after treatment of EMA were tested. The experimental results indicated that EMA has significant inhibitory effect on the growth of M. aeruginosa PCC7806, and the value of EC(50,7d) was 2.0 mg x L(-1). However, the inhibition declined with the cultivation time. During the whole cultivation period, EMA showed no significant effect on the release of MC-LR from cells to the culture. After 7 days, the amount of intracellular MC-LR per cell unit increased with the increasing of EMA concentration. The amount of MC-LR per cell unit was 25 ng x (10(6) cells)(-1) after the treatment with 1.5 mg x L(-1) EMA, which was increased by 39% compared with the control. The total MC-LR production (including intracellular and extracellular MC-LR) first slightly increased and then decreased significantly with the increase of EMA concentration. After the treatment with 3 mg x L(-1) EMA, the total MC-LR production was 28 microg x L(-1) (only half of that in the control). After 16 days, EMA showed no significant effect on both the amount of MC-LR per cell and the total MC-LR production.