D-Malate dehydrogenase (D-malate:NAD+ oxidoreductase (decarboxylating), EC 1.1.1.83) was purified to homogeneity from Pseudomonas fluorescens UK-1 grown on D-malate and some properties of the purified enzyme were compared with those of beta, beta-dimethylmalate dehydrogenase (3,3-dimethyl-D-malate:NAD+ oxidoreductase (decarboxylating), EC 1.1.1.84). D-Malate dehydrogenase has the molecular weight and subunit size of 140 000 and 34 000, respectively (the same as those of beta, beta-dimethylmalate dehydrogenase). The amino acid compositions of the two enzymes are similar as well. D-Malate dehydrogenase cross-reacted with anti-beta, beta-dimethylmalate dehydrogenase and beta, beta-dimethylmalate dehydrogenase cross-reacted with anti-D-malate dehydrogenase. The two dehydrogenases have similar catalytical properties. Both of the dehydrogenases were unaffected by sulphydryl reagents but were inactivated by 1,2-butanedione. NAD provided better protection against inactivation than D-malate or beta,beta-dimethyl-DL-malate.