Pollution of marine environment has become an issue of major concern in recent years. Serious environmental pollution by heavy metals results from their increasing utilization in industrial processes and because most heavy metals are transported into the marine environment and accumulated without decomposition. The aim of the present study is to investigate the effects on growth, pigments, lipid peroxidation, and some antioxidant enzyme activities of marine microalga Pavlova. viridis, in response to elevated concentrations of cobalt (Co) and manganese (Mn), especially with regard to the involvement of antioxidative defences against heavy metal-induced oxidative stress. In response to Co2+, lipid peroxidation was enhanced compared to the control, as an indication of the oxidative damage caused by metal concentration assayed in the microalgal cells but not Mn2+. Exposure of P. viridis to the two metals caused changes in enzyme activities in a different manner, depending on the metal assayed: after Co2+ treatments, total superoxide dismutase (SOD) activity was irregular, although it was not significantly affected by Mn2+ exposure. Co2+ and Mn2+ stimulated the activities of catalase (CAT) and glutathione (GSH), whereas, glutathione peroxidase (GPX) showed a remarkable increase in activity in response to Co2+ treatments and decreased gradually with Mn2+ concentration, up to 50 μmol/L, and then rose very rapidly, reaching to about 38.98% at 200 μmol/L Mn2+. These results suggest that an activation of some antioxidant enzymes was enhanced, to counteract the oxidative stress induced by the two metals at higher concentration.