An extracellular xylanase was purified to homogeneity from a wheat bran culture of the thermophilic fungus HG-1, an isolate from a compost heap. The enzyme had a molecular weight of 33,000 by SDS PAGE and 31,000 by gel filtration; its isoelectric point was 6.8. The optimum temperature and pH for enzyme activity were 70 degrees C and 4.5-5.0. The enzyme was stable in the pH range from 2 to 12 at 30 degrees C. The K-m values for birchwood xylan and oat-spelt xylan were 8.3 and 20 mg/ml, respectively. The enzyme produced xylobiose, xylotriose, and a trace of xylose as the endo products from birchwood xylan. The enzyme activity was strongly inhibited by SDS, and partially by Hg2+, Mn2+, Co2+, Ca2+, and iodoacetic acid. The enzyme hydrolyzed xylotriose to xylobiose and xylose and showed weak activity toward xylobiose.