Differences between xylanases produced by Aspergillus FP-470 growing under normal (37 degrees C) and temperature stress conditions (45 degrees C) were evaluated. Fungal growth at 45 degrees C was seriously affected. However, the xylanase specific activity was 2.5 times higher than that produced at 37 degrees C. Optimum pH and temperature were 6.5, 80 degrees C and 4.3, 50 degrees C for the xylanases produced at 37 degrees C and 45 degrees C, respectively. Electrophoresis showed that a more complex xylanolytic system was produced at 37 degrees C. Stress induced by temperature produced a decrease in the number of extracellular xylanases released by fungi, but increased the production of some xylanases, possibly essential for growth under stress conditions.