The extent to which sugars serve as targets for superoxide was examined using glycolaldehyde, as the simplest sugar, and using superoxide dismutase (SOD)-replete and null strains growing under aerobic and anaerobic conditions. Glycolaldehyde was more toxic to the SOD-null than to its SOD-replete parent and this differential effect was oxygen-dependent. The product, glyoxal, could be trapped in the medium by 1, 2-diaminobenzene and assayed as the quinoxaline. The SOD-null strain produced more glyoxal and eliminated it more slowly than the parent. Glyoxal was ~10-times more toxic than was glycolaldehyde and was more toxic to the SOD-null than to the parental strain. 1, 2-Diaminobenzene protected against the toxicity of glycolaldehyde. These E.coli strains contained the glutathione-dependent glyoxalases I and II as well as the glutathione-independent glyoxalase III. Of these, glyoxalase III was most abundant and it was inactivated within the aerobic SOD-null strain and also in extracts when exposed to the flux of superoxide and hydrogen peroxide imposed by the xanthine oxidase reaction. It thus appears that short chain sugars are oxidized by superoxide yielding toxic dicarbonyls, moreover, the defensive glyoxalase III is also inactivated by the oxidative stress imposed by lack of SOD, thus exacerbating the deleterious effect of sugar oxidation.