Abstract: The toxic effects of a PSP-producing strain of dinoflagellate Alexandrium tamarense(ATHK) on antioxidant system of Chinese shrimp Fenneropenaeus chinensis, an important mariculture species in China, were studied in this study.The crude toxin extracted from A. tamarense (ATHK) cells were intramuscularly injected into the Chinese shrimp. The dose injection was carried out only one time during the experiment, using extracted solution from 1.4?103 algae cells. Superoxide dismutase(SOD) activity,glutathione-S-transferase(GST) activity and malonaldehyde (MDA) contents were analyzed in hepatopancreas and gill tissues after 1 h, 3 h, 6 h,12 h, 24 h and 48 h exposure.The SOD activity and GST activity in hepatopancreas and gill tissues increased within 6 h. However, they were inhibited at 12h and 48h in gill. the malonaldehyde (MDA) contents in hepatopancreas was no distinct change except at 1h , but increased in gill with time prolonging. The results indicate that the crude toxin extracted from A. tamarense cells could induce reactive oxygen species(ROS),which might associate with the metabolism of extracted toxins, and could cause branchial lipid peroxidation and oxidative stress in Chinese shrimp by induction of MDA increase and SOD/ GST inactivation.