Studies of reciprocal allelopathy among organisms in (terres- trial) plant competition have been focused on the growth of the target organisms (usually algae and/or cyanobacteria). Macro- phyte growth has been rarely investigated. In our current study, we presented evidence that the macrophyte growth (e.g. N. minor) can be negatively influenced by high density of M. aeruginosa. It has been reported that M. aeruginosa at high density may produce allelochemicals such as microcystin. It might be toxin like this that caused an inhibition on the growth of N. minor, thus limited its al- lelopathic effect. We noticed that the reciprocal allelopathic intensity was correlated to the organism density/biomass. The allelopathic inhibition of macrophyte N. minor on M. aeruginosa at intermedi- ate density was more significant than that at low- and high- densities. The mechanisms underlying low- and high density conditions might be different: at the low density, M. aeruginosa may be not sufficient to stimulate the N. minor to produce and release allelochemicals that can inhibit the M. aeruginosa; at high density, the cyanobacteria itself may produce and release toxins inhibiting the growth of N. minor, as mentioned earlier. Our current results are in agreement with previous studies showing M. aeruginosa can be controlled by C. vulgaris in a density-dependent manner. Our results further confirmed that allelopathic interactions between the two different organisms are density-dependent, even though the nu- trient and the light supply are sufficient [4,5,14,15]. Our current studies suggest that the macrophyte N. minor might be a useful tool for controlling phytoplankton blooms, esp. cyanobacteria blooms in the early stage.