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Two extracellular xylanases produced by the thermotolerant fungus Aspergillus caespitosus grown in sugar cane bagasse were purified and characterized. Estimated molecular masses were 26.3 and 27kDa (xyl I); 7.7 and 17.7 kDa (xyl II) for gel filtration and SDS-PAGE, respectively. Optimal temperature for both xylanases was 50–55°C. Optimal pH was 6.5–7.0 for xyl I, and 5.5–6.5 for xyl II. The thermostability (T_half) at 55°C was 27.3min (xyl I) and >90min (xyl II). Xylanase activity was inhibited by several ions. β-mercaptoethanol activated 59 and 102% xyl I and xyl II activities, respectively. These enzymes preferentially hydrolyzed birchwood xylan, and the K_m and V_max values were 2.5mg/ml and 1679U/mg protein (xyl I), and 3.9mg/ml and 113U/mg protein (xyl II). The action of both xylanases mainly that of xyl II, on kraft pulp reduced kappa number and increased pulp viscosity.