Cultures of Scenedesmus opoliensis P. Richter, used as single-species test systems, were exposed to differently polluted water samples collected from distinct sectors of the Somes River, in order to identify physiological and biochemical markers suitable for a reliable bioindication of changes in the quality of aquatic environments. Algal cultures grown in Bold's basal nutrient medium were used as control, and another reference consisted of cultures exposed to 25 µM CdCl₂. Environmental stress exerted by water pollution induced changes in the antioxidative protection of algal cells. When only small amounts of hydrogen peroxide were generated, the activities of ascorbate peroxidase and of superoxide dismutase were enhanced, while the activity of these enzymes declined when hydrogen peroxide accumulated in cells. Vitamin C content of algal cells increased moderately, but significantly under water pollution. The chlorophylls to carotenoids ratio and net oxygen production decreased, while membrane lipid peroxidation increased in polluted water. Cell density was more affected than dry biomass. From among the parameters of induced chlorophyll fluorescence, potential quantum efficiency and vitality index exhibited the most sensitive changes in accordance with the pollution of aquatic environment. The above mentioned parameters of Scenedesmus opoliensis may be useful biochemical and physiological markers of water pollution.