The activities of superoxide dismutase (SOD) isoenzymes were measured in fourth instar larvae of Chironomus riparius Mg. Three types of superoxide dismutase were identified: Cu,Zn-SOD in hemolymph and postmitochondrial fraction; Mn-SOD in mitochondrial fraction and presumably Fe-SOD in postmitochondrial fraction. The latter could have an endosymbiotic or a parasitic origin. Extracellular and cytosolic SOD activities, especially Cu,Zn-SOD, tended to increase in the last phase of larval development, independently of protein or hemoglobin contents. This supposes that SOD activity in Ch. riparius larvae is probably activated at the end of fourth instar stage. Cu,Zn-SOD and Mn-SOD activities showed a significant increase under severe hypoxia and slight hyperoxia. Oxygen radical scavengers such as SOD may play a role in the increased tolerance of Ch. riparius to oxidative stress. These results suggest that the specific induction of some SOD isoenzymes could be used as a biomarker of environmental disturbance such as oxidative stress initiated by xenobiotics.