Monocrotophos (MCP) is an organophosphate pesticide widely used in India for controlling various pests. In this study, we evaluated the oxidative stress and genotoxic potential of MCP on the freshwater mussel Lamellidens marginalis (Lamarck) after 7 days exposure and repair of the damaged DNA after 4 days recovery. The bivalves were exposed to 5.25 mg/L of MCP for 7 days and then allowed to recover for 4 days in pesticide-free water. Increase in the levels of thiobarbituric acid reactive substances was recorded in the gill, muscle, foot and mantle tissues. Cellular antioxidant defences i.e. antioxidant enzyme activities like catalase, superoxide dismutase, glutathione reductase and glutathione-S-transferase were used as biomarkers of oxidative stress. Altered activities of antioxidant enzymes were observed after exposure. There was a significant recovery in the antioxidative enzymes in the tissues after the recovery period. To monitor genotoxicity of MCP, we used micronucleus and comet assay. Increase in Olive tail moment in the gill cells of exposed mussels as compared to that of control ones indicated significant DNA damage. Our findings suggest that the MCP-induced oxidative stress may be contributing partly to genotoxic damage of gill cells. Thus, these biomarkers are found to be useful in evaluating the toxicity of MCP in mussels.