A low molecular weight, alkaline-stable endoxylanase (XylB) was purified to homogeneity from solid-state culture of Aspergillus fischeri Fxn1. XylB had a molecular mass of 13 kDa which is the lowest of reported xylanases. Optimal activity was at pH 6 and 55 degreesC. XylB was stable from pH 4.5 to 10 and up to 60 degreesC. It was nonglycosylated. The apparent K-m and V-max values of XylB on birch wood xylan were 0.53 mg ml(-1) and 0.2 mmol min(-1) mg(-1), respectively. The activity of XylB was not inhibited by Cd2+, Zn2+ Co2+, EDTA, iodoacetamide, beta-mercaptoethanol and acetic anhydride but strongly inhibited by 10 rum of N-bromosuccinimide, Hg2+, Pb2+ and p-hydroxymercuric benzoate. XylB is an endoxylanase since it hydrolysed xylan resulting the formation of xylo-oligomers but not of xylose residues.