We examined cyanobacterial toxin production in response to direct exposure to an axenically cultured aquatic plant (Lemna japonica Landolt) using two toxic monoclonal strains of Microcystis aeruginosa Küzing (NIES strains 103 and 107). The patterns of nutrient concentrations were similar between the controls and treatments. The exposure to toxic M. aeruginosa inhibited growth in L. japonica, and exposure to the aquatic plant caused an increase in microcystin (MC) production and inhibited growth in M. aeruginosa. The respective biomasses of M. aeruginosa and L. japonica differed significantly between control and treatments (p<0.05). Both intracellular MC and extracellular MC productions were significantly higher in the treatment than in the control for both strains (p<0.01). Increased MC production in response to direct exposure to this aquatic plant could be an induced defense mediated by the release of allelochemicals from the plant. This study revealed reciprocal allelopathic responses between Microcystis and Lemna, which can be applied to the management of eutrophic waters as well as an important information concerning strategies for recovering eutrophic waters.