Glutathione content and glutahione‐dependent enzymes were measured in the liver of two fish species, gudgeon(Gobio gobio)and roach(Rutilus arcasii),from the river Bernesga (Spain) caught downstream and upstream of the waste site of several chemical industries. Animals from contaminated sites display a reduced glutathione concentration and a tendency to the decrease of glutathione S‐transferase activity. Glutathione peroxidase activity was significantly elevated only in the liver ofGobio gobioand glutathione reductase activity in that ofRutilus arcasii.Our data indicate that the glutathione system constitutes a sensitive biochemical indicator of chemical pollution. Relative changes of glutathione and glutathione‐dependent enzymes in both fish species suggest a different susceptibility to toxins.