Abstract: In order to explore the effects and mechanisms of harmful algae blooms on abalone, we investigated the acute toxicity of four different harmful algae (Karenia mikimotoi, Alexandrium catenella, Prorocentrum donghaiense and Aureococcus anophageffferens) on the survival of Haliotis discus hannai Ino and the activities of antioxidant enzymes.The results suggested that K.mikimotoi and A.catenella could lead the larvae, juveniles and adults of H.discus hannai Ino to death.The survival rates of larvae reduced to 4.7% in 96 h when exposed to K.mikimotoi culture with algal density of 5000/mL.The survival of larvae rates were 25.5% in 96 h when exposed to A.catenella culture at the cell amounts of 4000/mL, and the survival rates of juveniles and adults reduced to 3.3% and 6.7% in 48 h larvae at the cell amounts of 10000/mL.P.donghaiense and A.anophageffferens could lower the survival rates of larvae, but had no significant effect on that of juveniles and adults.Meanwhile, K.mikimotoi and A.catenella inhibited SOD and CAT activities in the gills, and SOD and GSH-Px activities in the hepatopancreas of abalone, which suggested that these two algae would damage the immune system of abalone.However, P.donghaiense and A.anophageffferens had no influence on the three antioxidant enzymes of abalone.Therein, this study revealed that K.mikimotoi and A.catenella blooms may be serious threats to abalone aquacultures, and P.donghaiense and A.anophageffferens may have potential negative impacts on the resources complement of abalone stacks.