Clostridium sp. strain X53, newly isolated from termites, was found to produce xylanase and hydrogen in a batch culture with xylan. The maximal enzyme activity of crude xylanase produced in the culture was obtained at 50 degrees C and pH 5.0. Xylanase activity was detectable in the culture medium (10 g/l of xylan) after 2 h, and reached a peak (1,252 IU/ml) after 8-h cultivation at 40 degrees C and pH 6.0. The most efficient production of hydrogen from xylan was coincidently achieved under the same conditions as xylanase production, at 40 degrees C and pH 6.0. The maximal hydrogen evolution rate and total hydrogen yield were 240 ml/l/h after 8-h cultivation and 1,254 ml/l for 24 h, respectively, although the amount of hydrogen produced from xylan was equivalent to about 73% of that from the 4.7 g of xylose consumed.