The anaerobic aerotolerant bacterium Zymomonas mobilis 113 produced superoxide (O2-) and hydrogen peroxide (H2O2) under aerobic conditions. The main generators of H2O2 were glucose oxidase and superoxide dismutase (SOD). The O2-generation was probably related to minor alternative reduced nicotinamide adenine zinucleotide (NADH)-oxidation reactions in the electron transport chain. An increase in medium pO2 was observed during growth of Z. mobilis 113 in a batch culture. The maximum pO2 increase correlated with glucose oxidase and SOD activities. An decrease in medium pO2 value coincided with an increase in catalase activity in batch culture. Medium deoxygenation reduced the pO2 effect, yet the culture still responded with a pO2 increase after inoculation and addition of the feeding medium. We conclude that the apparent pO2 effects are related to changes in H2O2 concentration in the culture liquid.