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¹ University of Calcutta Department of Zoology Aquatic Bioresource Research Laboratory, 35, Ballygunge Circular Road, Kolkata West Bengal - 700019, India DOI : 10.4194/1303-2712-v20_4_02 Viewed : 2735 - Downloaded : 2147 The study shows effect of oxidative stress on liver and spleen of L. rohita (Hamilton, 1822) challenged with an asymptomatic dose (3 × 10⁷ CFU/ mL) of Aeromonas hydrophila. Upon termination of the experiment, intracellular Reactive Oxygen Species (ROS), anti-oxidant enzyme activities of Super Oxide Dismutase (SOD) and Catalase (CAT), anti-oxidative enzyme concentrations of Glutathione Peroxidase (GPx), Glutathione Reductase (GR), Glutathione (GSH) were recorded where an organ specific pattern of significant changes were observed between control and treated samples. Study revealed apoptotic cell death in both organs where early and late apoptotic cells increased significantly. Mitochondrial membrane potential assay reflected higher percentage of depolarized cells in treated samples. Cytosolic free Ca²⁺ level was significantly higher in liver and spleen post infection and was also associated with significant increase in Caspase 3 gene expression. Evidences of severe oxidative stress in fishes resulting from an asymptomatic dose eventually causing apoptotic cell death were reported. It was observed to be mediated by effector Caspase 3 which was triggered by a complex interplay between mitochondrial dysfunction and high level of cytosolic Ca²⁺. Further, it indicated an intriguing observation that liver is more susceptible towards the pathogen and shows more significant changes in comparison to spleen. Keywords : Oxidative stress, Mitochondrial membrane Potential, Caspase 3, Apoptotic cell death