1. In membranes prepared from dark grown cells of Rhodopseudomonas capsulata, five cytochromes of b type (E'0 at pH 7.0 +413+/-5, +270+/-5, +148+/-5, +56+/-5 and -32+/-5 mV) can be detected by redox titrations at different pH values. The midpoint potentials of only three of these cytochromes (b148, b56, and b-32) vary as a function of pH with a slope of 30 mV per pH unit. 2. In the presence of a CO/N2 mixture, the apparent E'0 of cytochrome b270 shifts markedly towards higher potentials (+355mV); a similar but less pronounced shift is apparent also for cytochrome b150. The effect of CO on the midpoint potential of cytochrome b270 is absent in the respiration deficient mutant M6 which possesses a specific lesion in the CO-sensitive segment of the branched respiratory chain present in the wild type strain. 3. Preparations of spheroplasts with lysozyme digestion lead to the release of a large amount of cytochrome c2 and of virtually all cytochrome cc'. These preparations show a respiratory chain impaired in the electron pathway sensitive to low KCN concentration, in agreement with the proposed role of cytochrome c2 in this branch; on the contrary, the activity of the CO-sensitive branch remains unaffected, indicating that neither cytochrome c2 nor the CO-binding cytochrome cc' are involved in this pathway. 4. Membranes prepared from spheroplasts still possess a CO-binding pigment characterized by maxima at 420.5, 543 and 574 nm and minima at 431, 560 nm in C0-difference spectra and with an alpha band at 562.5 nm in reduced minus oxidized difference spectra. This membrane-bound cytochrome, which is coincident with cytochrome b270, can be classified as a typical cytochrome "0" and considered the alternative CO-sensitive oxidase.