1. Light-grown cells of a blue-green mutant of Rhodospirillum rubrum possess cytochrome c2, cytochrome cc', cytochrome b-557.5 and cytochrome B. Approximately 75% to 90% of the total amounts of cytochrome c2 and cytochrome cc' are present in the cytoplasmic fluid, the remainder being associated with the chromatophore membrane, which contains cytochrome B also. It is not known whether cytochrome B is the cytochrome b-557.5 that has been purified. 2. With chromatophores, the cytochrome cc' is reduced by succinate, whereas the cytochrome B is reduced by NADH. The Em7 values of the cytochromes at the associated state are +0.311 V (n = l) for cytochrome c2, +0.017 V (n = l) for cytochrome cc' and −0.16 V (n=2) for cytochrome B. 3. The absorption spectrum of cytochrome cc' is significantly different between the free and the associated state. At the associated state, it is rather similar to that of the typical b-type cytochrome. 4. One chromatophore of the average size possesses 790 molecules of bacterio-chlorophyll, 440 molecules of ubiquinone-10, 78 molecules of rhodoquinone, 5 molecules of cytochrome c2, 5 molecules of cytochrome cc', 5 molecules of cytochrome B, 360 atoms of non-heme iron, 2 molecules of FAD, 1 molecule of FMN, and 3, 100 atoms of lipid phosphorus.