Aspergillus terreus THOM produced appreciable yield of xylanase on medium containing acid pretreated rice straw as sole carbon source. The enzyme was purified approximately 25-fold by ammonium sulfate precipitation, gel filtration through Sephadex G-50 and ion-exchange chromatography on DEAE-cellulose with a yield of about 23% and specific activity of 15.38 units/mg protein. Optimum activity against xylan was at 45 degrees C and pH 4.5. Relative stability of the enzyme was recorded at pH range of 4-5.5. Heating the enzyme preparation at 60 degrees C for one hour resulted in a 82.61% loss of activity. After exposing to 90 degrees C for 10 minutes xylanase retained 4.28% of its original activity. The purified enzyme lost 25% of the original activity after keeping at 4 degrees C for 9 months in 0.05 M acetate buffer (pH 4.5). The Km value of the enzyme was found to be 0.83 mM. Zn2+ was the most enhancing agent for xylanase activity. Cu2+ followed by Co2+ and K+ were the more deterrent cations. Xylanolytic activity of A. terreus was strongly inhibited by HgCl2, 2,4-dinitrophenol (DNP), phloridzin and ethylene diamino tetra acetic acid (EDTA).