Novel polypeptides possessing (endo) xylanase activity are disclosed which can degrade cellulose in plant extracts and plant materials. The polypeptides can cleave .beta.-D-xylan polymers at internal (1-4) bonds between adjacent xylopyranosyl units. The amino acid sequence and encoding DNA sequence is given and the polypeptide was used to treat cellulose in the preparation of edible foodstuffs and animal feed. The polypeptides have both arabinoxylanase and xylosidase activity.