Abstract Chemotrophically grown Rhodopseudomonas viridis contains little bacteriochlorophyll b and no detectable reaction center. Membranes isolated from chemotrophically grown Rps. viridis appear to contain cytochrome d and cytochrome o terminal oxidases. The membranes also appear to contain a cytochrome bc 1 complex, since cytochrome c 1 ( M r = 31 000, detected by immunoblotting) and b-cytochromes are present. Chemotrophically grown Rps. viridis contains neither the membrane-bound, reaction center-associated cytochrome c-552 c-558 peptide nor any soluble cytochrome c 2. A previously undetected soluble b-type cytochrome, apparently absent in phototrophically grown Rps. viridis, was found in chemotrophically grown cells. Preliminary measurements indicate that cytochrome b-560 is probably a dimer of about 20 kDa subunits and has an oxidation-reduction midpoint potential of −20 mV.