Aerobic organisms possess an array of enzymatic defense mechanisms against the toxic effects of active oxygen species. These include CuZn superoxide dismutase (CuZn SOD), catalase (CAT), and glutathione peroxidase (GPOX). Insects, however, lack an independent GPOX enzyme and instead rely on the activity of the more general detoxification enzyme, glutathione-S-transferase (GST), to carry out a peroxidase function. We report here the developmental profile of GST in Drosophila melanogaster and show that GST is induced by paraquat, a known free-radical generating agent. We also report that glutathione (GSH) depletion induced by administration of buthionine sulfoximine (BSO) selectively reduces the viability of mutants lacking CuZnSOD. By measuring GST specific activity in flies carrying deficiencies for the 87B region, we confirm an earlier report that this region contains active GST-encoding genes. Finally, through a biochemical analysis of representative alleles of known lethal complementation groups in this region, we have identified ck17 as a putative GST subunit gene. The implications of these findings to the role of GSH and GST in D. melanogaster oxygen defense are discussed.Key words: glutathione-S-transferase, paraquat, CuZn superoxide dismutase, oxygen defense.