Rhodobacter capsulatus J1 has two hydroperoxidases: a catalase-peroxidase and a peroxidase. A mutant strain, AH18, that had no catalase-peroxidase was isolated. The growth rate under aerobic and photosynthetic conditions, respiration, superoxide dismutase and peroxidase activities, and pigment content of the mutant were similar to those of the wild type. AH18 was more susceptible to killing and to inhibition of nitrogenase by H2O2 but not by molecular oxygen. The incidences of spontaneous mutations were similar in both strains. Viable counts in aerobic but not anaerobic cultures of AH18 started to decline as soon as the cultures reached the stationary phase, and the rate of cell death was much higher in AH18 than in the wild type. It is inferred that the peroxidase provides protection against H2O2 in log-phase cells and that the catalase-peroxidase provides protection under the oxidative conditions that prevail in aging cultures. This protective function might be related to the dual activity of the latter as a catalase and a peroxidase or to its capacity to oxidize NADH, NADPH, and cytochrome c.