The xynA gene (encoding xylanase) from the obligately anaerobic thermophilic bacterium Caldocellum saccharolyticum has been inserted into the yeast expression vector, pFLAGU2. Yeast cells containing this vector were able to produce and secrete active xylanase into the growth medium. Xylanase was purified by the use of an affinity column specific for a rare peptide sequence fused to the N-terminus of the xylanase. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis of the purified fractions revealed that the enzyme had been fortuitously glycosylated. The specific activity of the purified xylanase was found to be 90 international units/mg protein. The amount of xylanase secreted into the surrounding medium was approximately 10 mg/l.