Studies on exposures of gills of green-lipped mussel Perna viridis to sublethal levels of mercury (Hg) indicates that oxidative stress marker like lipids peroxidation and protein carbonyl content increase. With the exception of superoxide dismutase, the activities of catalase, glutathionine peroxidase, glutathionine reductase and glutathione-S-transferase also showed higher values. Reduced glutathione and H sub(2) O sub(2) level gradually increased throughout the experimental period. Ascorbic acid content of the gill tissue was positively correlated with SOD activity. Similarly, a positive correlation was shown by lipid peroxidation and H sub(2) O sub(2) levels. The DNA strand breaks observed on the 5th day of the Hg exposure, regained later. These properties of gill tissue could effectively be utilized to measure sub lethal toxicity of Hg pollution during environmental monitoring studies