A model is proposed for the respiratory adaptation to falling oxygen concentration during growth of the microaerophilic bacterium Campylobacter mucosalis. During the early stages of growth, the oxidation of formate is a two-stage branched process involving the production of H2O2 followed by its peroxidatic removal. In later stages of growth, at lower oxygen concentrations, the predominant electron flow is linear to a membrane-bound cytochrome-c oxidase which reduces O2 directly to H2O. Several components of this model have been investigated. H2O2 was produced during formate oxidation and accumulated when electron transfer to the cytochrome-c peroxidase was inhibited. A cytochrome c-553, of the Class 1 types, was purified and shown to be the specific electron donor to both the peroxidase and the membrane-bound oxidase. The levels of this cytochrome c and of the peroxidase were higher in cells harvested early in growth. In later stages of growth, the activity of the membrane-bound oxidase increased. Proton pumping across the membrane was detected with either H2O2 or oxygen as terminal electron acceptor. The novel energy-conserving role of H2O2 in this catalase-negative bacterium is discussed in relation to its microaerophilic nature.