To investigate the potential allelopathic effect of Alexandrium tamarense on the growth of Prorocentrum donghaiense, experiments were designed to co-culture both algae or to culture P. donghaiense with cell-free filtrate from A. tamarense under N- and P-limited and nutrient-sufficient （normal） conditions. The results showed that the growth of P. donghaiense was inhibited when semi-continuously cultured in cell-free filtrate of A. tamarense. Under N-limited condition, cell density of P. donghaiense was significantly lower than the control （without addition of filtrate） after 5 days of culture, the cell density being 1.02×10⁷ cells L^-1, buf that of control was 1.7× 10⁷ cells L^-1. Under the P-limited condition, there was no obvious difference in cell density between treated alga and the control, the cell density of treated alga being 1.44× 10^7 cells L^-1. Under nutrient-sufficient condition, there was no difference in cell density between P. donghaiense and the control. In co-culture, the inhibitory effect ofA. tamarense on P. donghaiense was more obvious. Under N- and P-limitied conditions, P. donghaiense was entirely killed by A. tamarense after 4 days, the former gathered into groups and sank. However, a few P. donghaiense survived （cell density at 3.3 × 10⁴ cells L^-1） under nutrient-sufficient conditions. The results suggested that A. tamarense have allelopathic effect on P. donghaiense, and the production ofallelochemicals would be stimulated under N- and P-limited conditions. Thus, the inhibitory effect of A. tamarense might be one of the causes that suppresses the growth of P. donghaiense.