Multiple acidophilic xylanolytic enzymes were produced by Penicillium oxalicum GZ-2 during growth on wheat straw, rice straw, corn stover, and wheat bran. The expression of xylanase isoforms was dependent on substrate type and nitrogen source. The zymograms produced by the SDS-PAGE resolution of the crude enzymes indicated that wheat straw was the best inducer, resulting in the highest xylanase (115.2 U/mL) and beta-xylosidase (89 mU/mL) activities during submerged fermentation. The optimum temperature and pH for xylanase activity were 50 degrees C and 4.0, respectively; however, the crude xylanase enzymes exhibited remarkable stability over a broad pH range and showed more than 90% activity at 50 degrees C for 30 min at pH 4.0-8.0. The results revealed that P. oxalicum GZ-2 is a promising acidophilic xylanase-producing microorganism that has great potential to be used in biofuels, animal feed, and food industry applications. (C) 2012 Elsevier Ltd. All rights reserved.