A thermostable xylanase was isolated from an Aspergillus niger wild type strain in a liquid Czapek Dox medium, containing oat spelts xylan as the sole carbon source. The molecular mass of the enzyme was estimated to be about 36 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The optimum pH for activity was found to be 7.5. The temperature optimum of the enzyme was found to be 60 degreesC at pH 7.5. The enzyme remained stable up to 100 degreesC, yet lost about 50% of its activity after 15 min at this temperature.