Dye-linked ethanol dehydrogenases from Pseudomonas aeruginosa ATCC 17,933 and P. putida ATCC 17,421 were purified to homogeneity and crystallized. The amino acid composition of the two enzymes is very similar and the number of the aromatic amino acid residues found per subunit are almost identical. With respect to their catalytic and molecular properties both ethanol dehydrogenases are similar to the quinoprotein methanol dehydrogenases known from methylotrophic bacteria. They show a high pH-optimum, need ammonia or an amine as activator and are dimers of identical subunits of a molecular mass of 60,000. The dimer is the catalytically active form. Each subunit carries one prosthetic group pyrroloquinoline quinone, which can be titrated by the suicide substrate cyclopropanone ethylhemiketal. In contrast to the general methanol dehydrogenases the two ethanol dehydrogenases have a low affinity for methanol and in addition to primary alcohols they also oxidize secondary alcohols. With secondary alcohols preferentially one of the two enantiomers is oxidized. The catalytic and spectral properties of the two enzymes are very similar to the quinoprotein ethanol dehydrogenase isolated from P. aeruginosa LMD 80.53 (Groen et al., 1984. Biochem. J. 223: 921-924). However this enzyme is reported to be a monomer of molecular mass 100,000.