Aquatic reservoirs remain the ultimate sink of chemical pollutants emanating from anthropogenic activities such as agricultural, industrial and mining processes. Chemicals such as pesticides, heavy metals and polycyclic hydrocarbons are generated during the various processes carried out by man and these chemicals find their way to aquatic bodies via runoff, leaching and aerial drifts. Freshwater biota undoubtedly is at risk from the adverse effects of these water pollutants and there is therefore, a need to monitor effects of these chemical pollutants in order to safeguard the health of aquatic life. We investigated the oxidative stress effects of chlorpyrifos and lead on the freshwater snail Helisoma duryi to assess the potential of using this enzyme system as a biondicator of exposure to environmental pollutants. Groups of snails were exposed to 5 ppb lead acetate and 25 ppb chlorpyrifos for 7 days after which half of the snails were sacrificed and the other half allowed to recover in clean water and sacrificed after another 7 days. Post mitochondrial fractions were used to measure the activities of the following antioxidant enzymes; superoxide dismutase, catalase, glutathione peroxidase, glutathione S-transferase and diphosphotriphosphodiaphorase. Both pollutants enhanced the activities of all the antioxidant enzymes suggesting a defensive mechanism by the snail to combat the oxidative stress due to the organophosphate chlopryrifos and metal pollutant lead. There was a significant recovery of the antioxidant defense system of the snails allowed to recover in clean water shown by the reduced alteration of the antioxidant enzyme activities of the snails allowed to recover for 7 days. This supports the need to minimize exposure of aquatic biota to chemical pollutants and if polluted the need remediate the polluted water reservoirs in order to safe guard the health of aquatic organisms.