Xylanase production by a newly isolated Aspergillus foetidus MTCC 4898 was studied using submerged fermentation under shaking. The maximum xylanase yield (2 10 U/ml) was obtained with negligible cellulase activity at 30 degrees C using 1% birchwood xylan as substrate in 3 days of cultivation. The strain was able to produce extracellular xylanases on birchwood xylan as well as on cellulose rich substrates also with very poor cellulase production (< 0.1 FPU/ml). The partially purified enzyme recovered by ammonium sulphate fractionation showed maximum activity at 50 degrees C and at pH 5.3. The enzyme was highly stable at 40 degrees C but retained 36% of its activity at 50 degrees C after 3 h. The presence of KCl, CaCl2, glycerol, xylan and trehalose significantly improved thermostability at 50 and 60 degrees C. Zymogram analysis indicated three isoenzymes. The enzyme preparation was completely stable at freezing temperature for up to 6 months and reasonably stable at refrigeration temperatures for more than 2 weeks. (c) 2004 Elsevier Ltd. All rights reserved.