Aerobic organisms are protected against oxidative stress by antioxidant systems which mobilise enzymes such as the Cu/Zn superoxide dismutase (Cu/Zn-SOD) which transfers O2(.-) to H2O2. In this paper, we report the characterization of three isoforms of Cu/Zn-SOD in the blue mussel Mytilus edulis and we show that one of these isoforms is strongly inducible. Cytosolic extracts of digestive gland and gills from adult blue mussels were analysed by polyacrylamide gel electrophoresis or isoelectric focusing followed by in situ staining for SOD activity. Two main bands of Cu/Zn-SOD were obtained at pI 4.7 and 4.6 corresponding to native apparent molecular weight values of 205 and 155 kDa. Blue mussels from chemically contaminated area in Le Havre harbour exhibited a third Cu/Zn-SOD isoform characterized by a more acidic isoelectric point (pI 4.55) and a native apparent molecular weight of 130 kDa. When maintained in clean marine water, mussels from this area showed a transitory decrease in total SOD activity accompanied by the disappearance of the SOD-3 band. Conversely, the exposure (4 and 8 h, and 3 and 7 days) of control blue mussels to copper (25 microg l(-1)) markedly increased SOD-3 band while the total SOD activity did not systematically change. Taken together our results suggest that the variations of SOD expression pattern in Mytihus edulis could be used as a tool for the marine environment monitoring.