A xylanase has been found in the archaeon Thermococcus zilligii strain AN1 (DSM 2770), which grows optimally at 75°C. The enzyme had a molecular mass of 95 kDa and a unique N-terminal sequence. It had activity against all five xylans tested and against xylose oligomers, but not against other carbohydrate polymers. The K m values found for xylans were typical of those found for bacterial xylanases. The pH optimum for activity was pH 6, and the enzyme half-life at 100°C was 8 min. This is the first description of any archaeal hemicellulase.