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Broad relationships among bacteria can be identified by ribosomal RNA analysis, but the resulting groups may not be easily definable by phenotypic characteristics. This is exemplified by the genus Campylobacter, which consists of at least three separate groups that cannot be differentiated readily by phenotypic characteristics. Examination of the type strains of all Campylobacter species (except Campylobacter pylori), Wolinella recta, Wolinella curva, Bacteroides ureolyticus, and Bacteroides gracilis revealed that sheathed flagella occur only in species of rRNA group II (except W.succinogenes). This is helpful in differentiating this group. Campylobacters are microaerophilic: they can respire with oxygen but cannot grow at the full level of oxygen found in an air atmosphere (21% O₂). Although W. recta, W. curva, B. ureolyticus, and B. gracilis are closely related to the campylobacters of rRNA group I, they were thought to be anaerobes, incapable of oxygen-dependent growth and of respiring with O₂. However, the present study revealed that they are in fact microaerophiles. They exhibited oxygen-dependent growth but failed to grow at 21% O₂ and grew only very slightly under anaerobic conditions unless provided with electron acceptors such as fumarate and nitrate. They exhibited 0₂ uptake with H₂ or formate as electron donors (W. recta showed only a low O₂ uptake with H₂). Oxygen uptake was inhibited by KCN and 2-heptyl-4-hydroxyquinoline N-oxide. The organisms possessed membrane bound cytochromes (cytochromes b560 and C551-553, and a CO-binding cytochrome c), as well as soluble cytochrome C552 and CO-binding cytochrome c. The cytochromes were reduced by H₂ and formate as electron donors. Proton efflux from cells in anaerobic suspensions containing H₂ or formate occurred upon addition of a pulse of oxygen. With formate as the electron donor, H+/O ratios of W. curva, W. recta, B. ureolyticus, and B. gracilis