A high activity of meso-alpha-epsilon-diaminopimelate dehydrogenase was found in extracts of Bacillus sphaericus, Brevibacterium sp., Corynebacterium glutamicum, and Proteus vulgaris among bacteria tested. B. sphaericus IFO 3525, in which the enzyme is most abundant, was chosen to study the enzyme reaction. The enzyme was not induced by the addition of meso-alpha-epsilon-diaminopimelate to the growth medium. The reaction product was isolated and identified as alpha-amino-epsilon-ketopimelate by a comparison of the properties of its 2,4-dinitrophenylhydrazone with those of an authentic sample in silica gel thin-layer chromatography, absorption, infrared and proton nuclear magnetic resonance spectrometry, and elemental analyses. The alpha-amino-epsilon-ketopimelate formed enzymatically was decarboxylated by H2O2 to yield L-alpha-aminoadipate. This suggests that the amino group with D-configuration in the substrate is oxidatively deaminated; the enzyme is a D-amino acid dehydrogenase. L-alpha-Amino-epsilon-ketopimelate undergoes spontaneous dehydration to the cyclic delta1-piperideine-2,6-dicarboxylate. The enzyme reaction is reversible, and meso-alpha-epsilon-diaminopimelate was formed in the reductive amination of L-alpha-epsilon-ketopimelate.