~~Four mutants of Methylobacterium extorquens AM1 which were deficient in the production of soluble c-type cytochromes have been isolated and characterized. They were unable to grow on methanol, ethanol or methylamine and c-type cytochromes were not detected in cell extracts by spectroscopy or in haem-stained SDS-polyacrylamide gels. Western blotting demonstrated that the mutants did not contain either the apo-or the holo-forms of cytochrome cL, although trace amounts of holo-cytochrome cH were detected. The mutants had only trace levels of methylamine dehydrogenase activity, suggesting a role for at least one of the c-type cytochromes in the production of the active form of this enzyme. Two regions of the M. extovquens AM1 chromosome which encode genes required for the synthesis of c-type cytochromes were identified and we have shown that, although the four mutants were phenotypically identical, they represent at least three complementation groups.