An acidic xylanase from a culture filtrate of Aspergillus nidulans grown on oat-spelt xylan was purified to apparent homogeneity. The purified enzyme showed a single band on sodium dodecyl sulphate-polyacrylamide gel electrophoresis with a molecular mass of 34,000 Da and had an isoelectric point of approximately 3.4. The enzyme was a non-debranching endoxylanase highly specific for xylans. The xylanase showed an optimal activity at pH 6.0 and 56° C and had a Michaelis constant K_m of 0.97 mg oat-spelt xylan (soluble fraction) ml and a maximed reaction velocity (Vmax) of 1,091 mol min^–1 (mg^–1protein)^–1. Using polyclonal antibodies raised against the purified enzyme, the regulation of its synthesis has been studied. The xylanase production is repressed by glucose and induced by oat-spelt xylan, arabinoxylan, 4-O-methylglucurono-xylan, birchwood xylan and xylose.