The newly isolated thermophilic fungus, Paecilomyces themophila J18, when grown on a medium containing corncob (4%, w/v) at 50 degrees C for 5 days, produced 1470 U ml(-1) of xylanase. Xylanase was purified 2.4-fold to homogeneity with a recovery yield of 38.3%. It appeared as a single protein band on SDS-PAGE gel with a molecular mass of approximately 25.8 kDa. The xylanase was a glycoprotein with a neutral carbohydrate content of 21.0%. It had an optimum pH of 7.0, and was stable over pH 6.0-11.0. The optimal temperature of the xylanase was 75-80 degrees C and it was stable up to 75 degrees C at pH 7.0. Apparent K. values of the xylanase for birchwood, beechwood, soluble and insoluble oat-spelt xylans were 1.6, 2.4, 2.0 and 11.8 mg ml(-1), respectively. The xylanase hydrolyzed beechwood xylan and xylooligosaccharides to yield mainly xylotriose and xylobiose as end products, suggesting it is an endo-xylanase. This is the first report on the purification and characterization of a thermostable xylanase from Paecilomyces themophila. These findings in this study have great implications for the future applications of the xylanase. (c) 2005 Elsevier Inc. All rights reserved.