Enzymatic treatment of pine and birch kraft pulps with a xylanase preparation from a thermophilic anaerobic bacterium Dictyoglomus sp. strain B1 was studied in order to improve pulp bleachability. Maximal solubilization of pulp xylan was obtained at 90° C and pH 6.0–7.0. The enzyme was also active in the alkaline pH range; at pH 9.0 xylan hydrolysis was decreased by only 18% from the maximum at pH 7.0. The positive effect of xylanase pretreatment at 80° C and pH 6.0 or 8.0 on bleachability of pine kraft pulp was demonstrated. The brightness was increased by two ISO units in one-stage peroxide delignification, which corresponds well to values obtained with other enzymes at lower temperatures and pH values. Thus, the Dictyoglomus xylanase is well suited for pulp treatments at elevated temperatures in neutral and alkaline conditions.