A xylanase gene from Paecilomyces thermophila was functionally expressed in Pichia pastoris. The recombinant xylanase (xynA) was predominantly extracellular; in a 5 l fermentor culture, the total extracellular protein was 8.1 g l(-1) with an activity of 52,940 U ml(-1). The enzyme was purified to homogeneity with a recovery of 48 %. The recombinant xynA was optimally active at 75 degrees C, as measured over 10 min, and at pH 7. The enzyme was stable up to 80 degrees C for 30 min. It hydrolyzed birchwood xylan, beechwood xylan and xylooligosaccharides to produce xylobiose and xylotriose as the main products.