Bipolaris sorokiniana, a cereal fungal pathogen, showed xylanase activity when grown on minimal salt medium containing 2% of cell wall prepared from barley leaves as the sole carbon source. The most abundant of the xylanases, 1,4-β-D-xylan xylanohydrolase (EC 184.108.40.206), was purified to homogeneity by S-Sepharose chromatography, and its properties were determined. The induction of xylanase was highest on the 12th day after inoculation in shake flask culture. The purified xylanase had a molecular weight of 30 kDa and an isoelectric point of 9·5. The pH and temperature optima of the B. sorokiniana xylanase activity were 5·5 and 70°C respectively. The enzyme was stable over the pH range 5–10 but the stability towards temperature fell sharply above 55°.