Abstract: The toxicity effect of Karenia species (Karenia selliformis and Karenia umbella) on rotifer Brachionus plicatilis has been studied. The results showed that two strains of K. selliformis and K. umbella had significant toxic effect on B. plicatilis. The toxicities of the two species were positively correlated with cell density in a short test time (2 hours), with a coefficient (R²) of 0.96 and 0.92, respectively. The 2 h LD₅₀ of K. selliformis and K. umbella on B. plicatilis was 2750/mL and 14011/mL, respectively, indicating a more intense toxicity of K. selliformis. Comparison of the toxicity of different algal fractions (culture cells, filtrate and broken cells) showed that culture cells had intense effects, while the filtrate and broken cells of both species have no toxicity on B. plicatilis. The direct contact with culture cells is to be one of the key factors that resulted in the deaths of rotifers.