The carboxymethylcellulase activity concentrated from the extremely thermophilic anaerobe H173 was found to have a pH optimum of 6.5–7.0. The enzyme activity was stabilised by the addition of dithiothreitol and CaCl₂•2H₂O and was very stable at 80° C, retaining 77% of the initial activity after 120 min incubtation. At min and after 120 min only 3% of the initial activity remained. With the enzyme dissolved in buffer, glucose and cellobiose were formed from the hydrolosis of Avicel. In culture medium the Avicel-solubilising activity was insensitive to the presence of up tp 50 mm glucose and showed linear glucose accumulation over a period of days at 70° C. HPLC analysis established that glucose was the major end-product of hydrolysis in the culture broths.