Rhodococcus rhodochrous is an active soil bacterium belonging to the Nocardia group of high GC Gram-positive bacteria. It is rich in various enzymes and thus important in the industrial production of chemicals and bioremediation. In this work, the respiratory chain of this aerobic organism was investigated and characterized. Grown under highly aerobic conditions, the membrane fraction of R. rhodochrous cells only contained a-, b- and c-type cytochromes, suggesting that it is the cytochrome bcc-aa(3)-type pathway that mainly operates under these conditions. In contrast, the d-type cytochrome was also present under microaerobic conditions, indicating that the alternative pathway of the bd-type oxidase works in these circumstances. In addition, the results of H+/O ratio measurements indicate that these two pathways have different energy efficiencies. (C) 2010, The Society for Biotechnology, Japan. All rights reserved.