A thermophilic Bacillus circulans AB 16, isolated from a garbage dump produced novel extracellular cellulase-poor, thermostable xylanase in basal medium containing rice straw. The xylanase yield was enhanced more than two and a half fold in the presence of tryptone and other media modifications. The highest xylanase activity obtained in liquid culture was 55 IU/ml. Two xylanases Xyl A and Xyl B were purified to homogenity by Q Sepharose and Sepharose 6B chromatograph. Xyl A (M-r 30 000) had a pH optima of 6 and a temperature optima of 75-80 degrees C. while Xyl B (M-r 22 000) also had a pH optima of 6 but with a temperature optima of 65-70 degrees C. Both Xyl A and Xyl B had a pH optima of 6 but retained 46% activity at pH 8. Xyl A retained 70% activity at 65 degrees C, pH 9 and 2 h incubation while Xyl B retained 34% under the similar conditions. Both Xyl A and Xyl B showed 90% inhibition in activity by 1 mM Hg2+. The hydrolysis pattern of Xyl A yielded mainly xylobiose with lower amount of xylo-oligosaccharides and xylose while Xyl B yielded mainly higher oligosaccharides with lesser amounts of xylobiose and negligible amounts of xylose. The crude enzyme from B. circulans AB 16 showed higher chromophore release of 0.360 U as compared to Xyl A and Xyl B with a chromophore release of 0.115 and 0.069 units and thus would be more useful for pulp bleaching than purified xylanases. (C) 2000 Elsevier Science Ltd. All rights reserved.