The Bradyrhizobium japonicum cbb3-type cytochrome oxidase, which supports microaerobic respiration, is a multisubunit enzyme encoded by the genes of the fixNOQP operon. We investigated the contribution of the individual subunits to function and assembly of the membrane-bound complex. In-frame deletion mutants of fixN, fixO, and fixQ, and an insertion mutant of fixP were constructed. All mutants, except the fixQ mutant, showed clearly altered absorption difference spectra of their membranes and decreased oxidase activities, and they were unable to fix nitrogen symbiotically. The presence of the individual subunits was assayed by Western blot analysis, using subunit-specific antibodies, and by heme staining of the c-type cytochromes FixO and FixP. These analyses led to the following conclusions: (i) FixN and FixO are necessary for assembly of the multimeric oxidase, (ii) FixN and FixO assemble independently of FixP, and (iii) FixQ is not required for complex formation and, therefore, does not seem to be an essential subunit. The possible oxidase biogenesis pathway involves the formation of a primary core complex consisting of FixN and FixO, which allows the subsequent association with FixP to form the complete enzyme.