We have analyzed the respiratory chains in the log-arithmic and stationary growth phases of Bacillus subtilis cells grown in rich glucose medium. The cytochrome c branch of the respiratory chain was absent from both types of cells, which used a quinol oxidase branch for respiration. Cytochrome aa3-600 was found to be the major terminal oxidase in log phase cells. This enzyme was shown to translocate protons across the membrane in addition to the charge separation in the oxidation of quinol. Both cytochromes d and aa3-600 were expressed in the stationary phase. After inhibition of the latter by cyanide, cytochrome d was shown to catalyze charge separation during quinol oxidation, but not to pump protons across the membrane. A CO-binding membrane-bound cytochrome of approximately 17 kDa, called cytochrome b558, was presented in log phase cells. This protein did not exhibit oxidase activity and did not have the characteristics of members of the conserved terminal oxidase family.