The present study was undertaken to evaluate the toxic effects of monocrotophos, a widely used organophosphorus pesticide, on Lamellidens marginalis with a wide battery of biomarkers consisting of AchE inhibition, lipid peroxidation, the levels of antioxidant enzymes, and histopathological changes. Animals were exposed to monocrotophos (52.36 mg/l) for four days. Malondialdehyde (MDA) values were measured as index of oxidation while Superoxide dismutase (SOD), Catalase (CAT), Glutathione s-Transferase (GST), and Glutathione-Reductase (GR) were measured as index of an antioxidant status. After exposure, a significant reduction of the capability to neutralize radicals was observed. Histopathological changes, such as fibrosis in gill filaments and hypertrophy in mucous cells of foot tissue, were observed after treatment. In a second series of experiment, exposed animals were thereafter transferred to clean water and kept in it up to 28 days to assess the recovery pattern. Significant recovery is observed in AchE and antioxidant enzymes. Oxidative damage observed after acute exposure indicate that mussels faced an oxidative challenge but were able to counteract, as values of anti-oxidants returned near to control values after 28 days. Altered activities in anti-oxidant enzymes due to stress recovered well after 28 days in gill and muscles as compared to foot and mantle. Overall results suggested that oxidative markers are highly sensitive and could be profitably applied to freshwater mussels for environmental quality assessment in freshwater.