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 <i>Campylobacter</i>, 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 <i>Campylobacter pylori), Wolinella recta, Wolinella curva, Bacteroides ureolyticus</i>, and <i>Bacteroides gracilis</i> revealed that sheathed flagella occur only in species of rRNA group II (except <i>W.succinogenes</i>). 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 <i>W. recta, W. curva, B. ureolyticus</i>, and <i>B. gracilis</i> 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 (<i>W. recta</i> 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 <i>W. curva, W. recta, B. ureolyticus</i>, and <i>B. gracilis</i> were 0.75, 1.66,2.06, and 2.04, respectively. With H₂ as the electron donor, H⁺/O ratios of <i>W.curva, B. ureoyticus, and B. gracilis</i> were 1.25, 1.97, and 2.36, respectively; technical difficulties prevented measurement of the ratio in <i>W. recta</i>. Proton translocation was inhibited by the protonophore carbonylcyanide <i>m</i>-chlorophenylhydrazone. The results confirm the relationship of these organisms to campylobacters. / Ph. D.