The thermophilic, xylanolytic, anaerobic organism, Dictyoglomus sp. B1, was cultivated in batch and continuous cultures in media containing insoluble beech-wood xylan. The extracellular xylanase activity levels obtained for the two cultivation methods were compared. Experiments were performed separately to determine the optimum substrate concentration, dilution rate, pH and temperature for xylanase production. Maximum xylanase activity was found at a substrate concentration of 1.5 g xylan/l, a dilution rate of 0.112 h(-1), pH 8.0 and at 73 degrees C. Different combinations of these optimum values were used in a 2(3) factorial experiment to investigate whether an increase in the xylanase production/activity could be achieved. A maximum xylanase activity of 2312 U/l was found when fermenters were operated at 73 degrees C with a substrate concentration of 1.5 g xylan/l, pH 8.0, and a dilution rate of 0.112 h(-1). Thus, the optimum xylanase activity in the factorial experiment was obtained when the conditions that gave the maximum xylanase activities in the individual experiments were combined. Optimum xylanase activity obtained in the 2(3) factorial experiment was 6.2 times higher than the activity found in the initial batch culture (373 U/l) and 3.0 times higher than the activity of a batch culture (783 U/l) grown at the same optimum conditions as the factorial experiment. The higher specific xylanase activity (217 U/mg protein) found in the 2(3) factorial experiment was 4.1 times higher than the specific activity in the initial batch culture (53 U/mg protein).