Oxidative stress and antioxidant defence systems were assessed in a marine brown alga Padina tetrastromatica, commonly occurring from the tropics. Lipid peroxidation (LPX) and H sub(2) O sub(2) were measured as oxidative stress markers, and antioxidant defences were measured as catalase (CAT), glutathione S-transferase (GST) and ascorbic acid (AsA), in order to understand their dissimilarity with respect to pollution levels from selective locations along the central west coast of India. Significant increased levels of LPX, H sub(2) O sub(2), CAT and GST were observed in samples from relatively polluted localities (Colaba and Karwar, India) when compared to less polluted locality (Anjuna), while AsA concentration was higher in algal samples from worst polluted region of Colaba. Heavy metals such as Cd and Pb were also higher in the vicinity of polluted areas compared to reference area. Variation of oxidative stress indices in response to accumulation of heavy metals within P. tetrastromatica could be used as molecular biomarkers in assessment and monitoring environmental quality of ecologically sensitive marine habitats.