A free-living nitrogen fixing Arcobacter nitrofigilis exhibuted the typical characteristics of aerobic growth in which the maximal cell growth was shown under an ambient air atmospjere, whereas no cell growth was shown umder an anaerobic condition. When oxygen concentration was increased, the activities of SOD and catalase were increased. These suggest that the aerobic nature of A. nitrofigilis might be due to the increased levels of both enzymes that scavenge toxic forms of oxygen.