D-Xylose (D-glucose) isomerase was purified to homogeneity in yields of approx. 1 g/kg of wet cells from a strain of Arthrobacter that produces it as about 10% of total soluble protein. It is a tetramer of identical 43,114 Da subunits containing a preponderance of acidic residues and no cysteine. Partial protein sequences were determined as a step to gene cloning. It requires Mg2+, Co2+ or Mn2+ for activity, Mg2+ being best; Ca2+ is an inhibitor, competitive with Mg2+. It is a good D-glucose isomerase with kcat. 1200 min-1 at pH 8 at 60 degrees C, which is higher than that of any other enzyme of this class. L-Arabinose, D-ribose and D-lyxose are poor substrates, with kcat. 78, 31 and 3.7 min-1 respectively at pH 8 at 30 degrees C, compared with 533 min-1 for D-xylose. Xylitol is a true competitive inhibitor for D-xylose (Ki 0.3 mM), but D-sorbitol shows mixed inhibition (Ki 6.5 mM). For D-fructose the pH optimum at 60 degrees C is 8, and at pH 7 the Arrhenius activation energy is 75 kJ/mol over the range 30-70 degrees C.