Xylanase was produced by Aspergillus terreus cultivated on finely ground wheat straw in solid-state fermentation. The optimal medium composition was developed by applying the Plackett-Burman experimental design. Best enzymic activity was obtained in a medium containing 10 g wheat straw/flask moistened with a concentrated nutrient salt solution to 75% initial water content and incubated for 4 days at 30 degrees C. A. terreus xylanase was fractionated by ammonium sulfate precipitation and purified by chromatography on DEAL Bio-Cel A followed by gel-filtration on Sephadex G-75. The enzyme was characterized by apparent V-max and K-m values of 333.3 U/mg protein and 16.7 mg xylan/ml, respectively, obtained for xylanase with oat spelt xylan as substrate. The optimal pH and temperature for maximum activity were 7 and 50 degrees C, respectively. The enzyme showed high specificity towards oat spelt xylan and minute activities were observed with carboxymethyl cellulose and cellobiose. About 48.02% of the activity remained after the enzyme had been incubated at 60 degrees C for 30 min. Metal ions such as Hg2+, Cu2+, Co2+, Fe3+, Pb2+ strongly inhibited xylanase, whereas, Ca2+ activated the enzyme.