Recently the interactions between zooplankton and Microcystis species have been concerned.Some studies have revealed that zooplankton can induce the change of toxin production in Microcystis.Microcystins(mcys) are cyclic heptapeptides, synthesized via a mixed polyketide synthase/nonribosomal petide synthetase system encoded by microcystin synthatase gene (mcy) operon, however no studies were conducted to elucidate the effect of zooplankton on the gene expressions of mcy genes.The present paper studied the effect of Daphnia upon the transcriptional regulation of genes responsible for microcystin synthesis in Murocystis aeruginosa PCC7806 using the real-time RT-PCR for the first time.Both mcyB and mcyD transcript levels increased under the grazing stress of Daphnia, and it is speculated that the increase of mcy gene clusters transcription was the induced defense of Microcystis aeruginosa PCC7806 against Daphnia stress.