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A mesophilic and three thermophilic clostridial species were newly isolated as potent xylan utilizing bacteria from mesophilic and thermophilic methane sludge, respectively. The mesophilic strain, X-4, had a characteristic fermentation pattern of xylan, producing ethanol and acetic acid in addition to hydrogen and carbon dioxide. The thermophiles, XY-6, XY-10 and XY-23, were strains of the typical butyric acid fermentation type. The utilization of xylan by mesophilic strain X-4 was evident at late exponential or early stationary growth phase followed by an increase in production of xylanase in the culture fluid, while xylan was fermented by the thermophilic strains from the beginning of their cultivation. Cell-bound xylanase was detected in the three thermophilic strains. Enzyme secreted into the medium (supernatant xylanase) was detected in strains XY-6 and XY-10 but not in strain XY-23. The actions of xylanase preparations from X-4 and XY-6 were characterized, indicating that cell-bound β-xylosidases were also involved in the degradation of xylan by both strains.