The accumulation of heavy metals in the body organs of aquatic organisms poses serious impacts on the activities of antioxidant enzymes. These enzymes are effective in protecting the bio-molecules against the reactive oxygen species (ROS) and their harmful effects. This study was aimed to evaluate the effects of sub-lethal concentrations of manganese on antioxidant activity in the fish, Labeo rohita. Fish were acclimatized for two weeks in the cemented tanks prior to the start of experiment. Four groups (n=10) of one year old fingerlings of Labeo rohita were exposed to 96-hr LC50 and sub-lethal concentrations (2/3rd,1/4th and 1/5th of LC50) of manganese for a duration of 30 days in the glass aquaria of 70L water capacity. After 30-day exposure, antioxidant (peroxidase) activity was measured in the fish liver, kidney, gills and brain and then compared with control group. The results showed that the antioxidant activity increased significantly in all organs of metal stressed fish as compared to the control fish. Among all test concentrations, the activity of antioxidant enzyme (peroxidase) was significantly higher in the fish liver under 96-hr LC50 manganese exposed fish as evident from its mean value as 0.685±0.005UmL-1, followed by 2/3rd (0.585±0.006 UmL-1), 1/4th (0.480±0.005 UmL-1), 1/5th (0.367±0.007 UmL-1) and control (0.278±0.005 UmL-1) group. Fish liver showed significantly higher (0.479±0.005 UmL-1) activity of peroxidase than that of other organs like kidney (0.396±0.006UmL-1), gills (0.356±0.004UmL-1) and brain (0.278±0.004UmL-1) of Labeo rohita.