D-xylose isomerase was cloned and characterized from a newly isolated actinobacteria strain, Thermobifisa fusca MBL 10003. There was only one base difference between the xylose isomerase genes from T. fusca XY, a cellulosic bacteria, and T. fusca MBL10003. The structural gene ( xyl A) is predicted to encode a polypeptide of 387 amino acids with an estimated molecular mass of 43,900. The deduced amino acid sequence of the gene showed high identity with homologous enzymes from the species of Streptomyces , Actinoplanes and Arthrobacter . The optimal temperature and pH were 75°C and 10, respectively. The enzyme was stable between 70 and 75°C, and between pH 4 and 12. Unlike other known xylose isomerases, the T. fusca enzyme had very similar K _m values for xylose and glucose, 264 and 274 mM, respectively. In contrast, k _cat and, therefore, k _cat/ K _m for xylose was approximately 18-fold larger than that for glucose. It required divalent metal ions for activity. Of these, Mg²⁺ was the most effective activator, and Co²⁺ functioned as a co-activator of Mg²⁺.