An amino acid racemase, which occurs in the cytoplasmic fraction of Aeromonas punctata subsp. caviae, has been purified to homogeneity by the criteria of electrophoresis and ultracentrifugation. The enzyme has a molecular weight of about 80,000 and consists of two subunits identical in molecular weight (about 40,000). The enzyme contains 2 mol of pyridoxal 5'-phosphate per mol of enzyme, and exhibits absorption maxima at 280 nm and 420 nm. The holoenzyme is resolved by dialysis against hydroxylamine to yield the inactive apoenzyme, which is reconstituted by the addition of pyridoxal 5'-phosphate to recover the full activity. The enzyme catalyzes racemization of a number of amino acids, e.g. lysine, ornithine, ethionine, arginine, glutamine, and methionine. The Michaelis constants were determined: 1 mM for L-lysine; 0.9 mM for D-lysine; 0.9 mM for L-ornithine; 1 mM for L-arginine; and 2.6 microM for pyridoxal 5'-phosphate. This enzyme is similar in enzymological properties to the racemase of Pseudomonas putida, but is distinct from it in immunochemical properties.