Evidence is presented for the sequence of 249 amino acids in ribitol dehydrogenase-A from Klebsiella aerogenes. Continuous culture on xylitol yields strains that superproduce 'wild-type' enzyme but mutations appear to have arisen in this process. Other strains selected by such continuous culture produce enzymes with increased specific activity for xylitol but without loss of ribitol activity. One such enzyme, ribitol dehydrogenase-D, has Pro-196 for Gly-196. Another, ribitol dehydrogenase-B, has a different mutation.