Xylanase production by seven fungal strains was investigated using concentrated spent sulphite liquor (SSLc), xylan and D-xylose as carbon substrates. An SSLc-based medium induced xylanase production at varying levels in all of these strains, with Aspergillus oryzae NRRL 3485 and Aspergillus phoenicis ATCC 13157 yielding activities of 164 and 146 U ml(-1), respectively; these values were higher than those obtained on xylan or D-xylose with the same fungal strains. The highest xylanase activity of 322 U ml(-1) was obtained with Aspergillus foetidus ATCC 14916 on xylan. Electrophoretic and zymogram analysis indicated three xylanases from A. oryzae with molecular weights of approximately 32, 22 and 19 kDa, whereas A. phoenicis produced two xylanases with molecular weights of about 25 and 21 kDa. Crude xylanase preparations from these A. oryzae and A. phoenicis strains exhibited optimal activities at pH 6.5 and 5.0 and at 65 and 55 degrees C, respectively. The A. oryzae xylanolytic activity was stable at 50 C over the pH range 4.5-10. The crude xylanase preparations from these A. oryzae and A. phoenicis strains had negligible cellulase activity, and their application in the biobleaching of hardwood pulp reduced chlorine dioxide consumption by 20-30% without sacrificing brightness.