Abstract: We studied the toxic effect of mequindox on growth inhibition, chlorophyll a content, total superoxide dismutase (T-SOD) activity and malondialdehyde (MDA) content of Nitzschia closterium f. minutissima and Isochrysis galbana Parke 3011, so as to provide basic data for safety evaluation of mequindox and protection of aquaculture environment.The results show that mequindox had acute toxicity effect on the growth of the two marine microalgae.The 24 h-EC₅₀ values of mequindox to N.closterium f. minutissima and I.galbana Parke 3011 were 4.73 mg · L^-1 and 2.14 mg · L^-1, respectively.The I.galbana Parke 3011 was more sensitive.After 24-hour exposure of mequindox, the chlorophyll a content of N.closterium f. minutissima decreased with increasing mequindox concentration significantly but no adverse effect was found on I.galbana Parke 3011.The T-SOD activity increased significantly with increasing mequindox concentration and the MDA content in all the experimental groups were higher than that in the control group, and I.galbana Parke 3011 was more sensitive.It is suggested that mequindox can cause oxidative stress and oxidative damage to cells of the two marine microalgae.