A strain of Aerobacter aerogenes PRL-R3 has been isolated which utilizes d-apiose as its sole source of carbon. A new enzyme, d-apiose reductase, was discovered in this strain. The enzyme was not present when the strain was grown on d-glucose. d-Apiose reductase catalyzes the nicotinamide adenine dinucleotide-dependent interconversion of d-apiose and d-apiitol. The enzyme is specific for d-apiose and d-apiitol, with a few possible exceptions. The K(m) for d-apiose is 0.02 m. The K(m) for d-apiitol is 0.01 m. The enzyme is almost completely specific for the reduced and oxidized forms of nicotinamide adenine dinucleotide. When cell-free extracts were centrifuged at 100,000 x g for 1 hr, the enzyme remained in solution. Optimal activity for the reduction of d-apiose was obtained at pH 7.5 in glycylglycine buffer, whereas for the oxidation of d-apiitol it was obtained at pH 10.5 in glycine buffer. Enzymatic reduction of d-apiose was not appreciably affected by the presence of 0.02 m ethylenediaminetetraacetate. Paper chromatography and specific spray reagents were used to identify d-apiitol and d-apiose as the products of this reversible reaction. d-Apiose and d-apiitol did not serve as substrates for ribitol dehydrogenase and d-arabitol dehydrogenase from A. aerogenes PRL-R3.