An extracellular thermostable xylanase produced by Saccharopolyspora pathumthaniensis S582 was purified 167-fold to homogeneity with a recovery yield of 12%. The purified xylanase appeared as a single protein band on SDS-PAGE, with a molecular mass of 36 kDa. The optimal temperature and pH of the xylanase were 70 °C and 6.5. The enzyme was stable within a pH range of 5.5-10.0. It retained its activity after incubation at 50 °C for 2 h. Its half lives at temperatures of 60 and 70 °C were 180 and 120 min respectively. Hydrolysis of beechwood xylan by the xylanase yielded xylobiose and xylose as major products. The enzyme acted specifically on xylan as an endo-type xylanase, and exhibited a K(m) value of 3.92 mg/mL and a V(max) value of 256 µmol/min/mg. Enzyme activity was completely inhibited by Hg(2+), and was stimulated by Rb(+) and Cs(+). The xylanase gene was cloned from genomic DNA of Saccharopolyspora pathumthaniensis S582 and sequenced. The ORF consisted of 1,107 bp and encoded 368 amino acid residues containing a putative signal peptide of 23 residues. This xylanase is a new member of family (GH) 10 that shows highest identity, of 63.4%, with a putative xylanase from Nocardiopsis dassonvillei subsp. dassonvillei.