The deduced amino acid sequences of four open reading frames identified upstream of the fixGHI region in Azorhizobium caulinodans are very similar to the putative terminal oxidase complex coded by the fixNOQP operons from Rhizobium meliloti and Bradyrhizobium japonicum. The expression of the A. caulinodans fixNOQP genes, which was maximal under microaerobiosis, was positively regulated by FixK and independent of NifA. In contrast to the Fix- phenotype of B. japonicum and R. meliloti fixN mutants, an A. caulinodans fixNO-deleted mutant strain retained 50% of the nitrogenase activity of the wild type in the symbiotic state. In addition, the nitrogenase activity was scarcely reduced under free-living conditions. Analysis of membrane fractions of A. caulinodans wild-type and mutant strains suggests that the fixNOQP region encodes two proteins with covalently bound hemes, tentatively assigned to fixO and fixP. Spectral analysis showed a large decrease in the c-type cytochrome content of the fixN mutant compared with the wild type. These results provide evidence for the involvement of FixNOQP proteins in a respiratory process. The partial impairment in nitrogen fixation of the fixN mutant in planta may be due to the activity of an alternative terminal oxidase compensating for the loss of the oxidase complex encoded by fixNOQP.