The mode of action of a minor xylanase on a variety of polysaccharides and model substrates was investigated. The enzyme was excreted by Thermoascus aurantiacus grown in solid state fermentation (SSF). The purified enzyme had a molecular mass of 33,000. Thin laser chromatography analysis showed that the endo-xylanase liberated short fragments from polysaccharides. The enzyme hydrolysed aryl-beta-D-cellobioside and the chromogenic (fluorogenic) 4-methylumbelliferyl-beta-glycosides of xylobiose (MeUmbXyl(2)) and xylotriose (MeUmbXyl(3)) at the agluconic Linkage. The results suggested that the endoxylanase belonged to family 10.