Thermophilic Humicola sp. secreted thermostable xylanases when grown on wheat bran medium at 50-degrees-C. DEAE-Sephadex A-50 column chromatography of the crude xylanase separated three fractions of xylanase (I, II and III), xylanase I being homogeneous in polyacrylamide gel electrophoresis after CM-Sephadex column chromatography. The respective xylanases, including the crude xylanase, increased pulp brightness but xylanases II and III decreased the viscosity of the pulp due to CMCase activity. The crude xylanase contained lower CMCase activity than xylanases II and III.