A ubiquinol-cytochrome c oxidoreductase (cytochrome bc1) complex has been purified from the plasma membrane of aerobically grown Paracoccus denitrificans by extraction with dodecyl maltoside and ion exchange chromatography of the extract. The purified complex contains two spectrally and thermodynamically distinct b cytochromes, cytochrome c1, and a Rieske-type iron-sulfur protein. Optical spectra indicate absorption peaks at 553 nm for cytochrome c1 and at 560 and 566 nm for the high and low potential hemes of cytochrome b. The spectrum of cytochrome b560 is shifted to longer wavelength by antimycin. The Paracoccus bc1 complex consists of only three polypeptide subunits. On the basis of their relative electrophoretic mobilities, these have apparent molecular masses of 62, 39, and 20 kDa. The 62- and 39-kDa subunits have been identified as cytochromes c1 and b, respectively. The 20-kDa subunit is assumed to be the Rieske-type iron-sulfur protein on the basis of its molecular weight and the presence of an EPR-detectable signal typical of this iron-sulfur protein in the three-subunit complex. The Paracoccus bc1 complex catalyzes reduction of cytochrome c by ubiquinol with a turnover of 470 s-1. This activity is inhibited by antimycin, myxothiazol, stigmatellin, and hydroxyquinone analogues of ubiquinone, all of which inhibit electron transfer in the cytochrome bc1 complex of the mitochondrial respiratory chain. The electron transfer functions of the Paracoccus complex thus appear to be similar, and possibly identical, to those of the bc1 complex of eukaryotic mitochondria. The Paracoccus bc1 complex has the simplest subunit composition and one of the highest turnover numbers of any bc1 complex isolated from any species to date. These properties suggest that the structural requirements for electron transfer from ubiquinol to cytochrome c are met by a small number of peptides and that the "extra" peptides occurring in the mitochondrial bc1 complexes serve some other function(s), possibly in biogenesis or insertion of the complex into that organelle.