Two of six tested strains ofHyphomicrobiumrespired on benzaldehyde with higher rates than on formaldehyde, and three strains with equal or lower rates, whereas one strain,HyphomicrobiumX, showed almost negligible respiration on benzaldehyde. Various substituted benzaldehydes stimulated oxygen consumption to lower rates than benzaldehyde itself in active strains. All strains contained multiple forms of dye-linked aldehyde dehydrogenase, as evident from polyacrylamide gel electropherograms of cell-free extracts and activity tests on the gels with nitroblue tetrazolium. All bands of this enzyme reacted more strongly with benzaldehyde than with formaldehyde in all strains. On gels of some strains additional bands appeared with benzaldehyde as the enzyme substrate.HyphomicrobiumX again displayed the lowest activity of this enzyme on the gels. A new band of this enzyme appeared on gels of strain ZV 580 after growth on methylamine, when tested in this respect. NAD-dependent secondary alcohol dehydrogenase was present on gels of three strains, which respired on 2-propanol.Difference spectra and observation of the degree of reduction of ubiquinone and cytochromesbandcof two strains indicated that the electrons from benzaldehyde oxidation were transferred to cytochromec.The results are discussed with regard to the significance of dye-linked aldehyde dehydrogenases with broad substrate specificity for the catabolism and oligocarbophilic growth ofHyphomicrobiumand the taxonomic relevance of the aldehyde dehydrogenase pattern observed on polyacrylamide gel electropherograms.