Expression of superoxide dismutase (SOD) isoforms was assessed as a biochemical marker, allowing to estimate the toxicity of chemicals toward a population of microalga Desmodesmus subspicatus (Chlorophyta). The effect of low (EC₁₀) and elevated (EC₅₀) doses of cadmium chloride (Cd), anthracene (ANT), and chloridazon (CHd) on SOD activity and isoform profile, as well as the amount of biosynthesized protein of ascribed to chloroplast (Fe-SOD) and cytosol (Cu/Zn-SOD) superoxide dismutase, were analyzed. Activities of Mn- SOD, Cu/Zn-SOD, and two Fe-SOD isoforms were determined densitometrically by native polyacrylamide gel electrophoresis and specific staining for SOD activity. Fe-SOD and Cu/Zn-SOD protein amount was analyzed by the semi-quantitative blot technique. The toxicants tested had no effect on the activity of Fe-SOD 1 isoform, whereas Fe-SOD 2 activity increased significantly. Simultaneously, the amount of Fe-SOD 1 protein increased after low contaminant doses treatment, whereas the amount of Fe-SOD 2 increased in cells exposed to elevated Cd, ANT, and CHd concentrations. A significant increase in Cu/Zn-SOD protein amount was observed in cells treated with EC₅₀ doses of ANT and Cd, as well as CHd in concentrations corresponding to the EC₁₀ value. At the same time, the activity of Cu/Zn-SOD increased in D. subspicatus cells exposed to an elevated dose of CHd. We have found that in the cells of D. subspicatus, the induction of Fe-SOD 1 and Cu/Zn-SOD Csd2 biosynthesis responds to xenobiotic-mediated stress level. The increase in the amount of chloroplastic Fe-SOD 1 and Cu/Zn-SOD Csd2 isoforms may be a sensitive biochemical marker of stress conditions, revealing the effects of low doses of aquatic contaminants on green algae.