From enrichment cultures on cellulose and xylan at 65°C we have purified 39 thermophilic strains of strict anaerobes displaying the main morphological characteristics of the genus Clostridium, and producing acetate, ethanol and lactate. Initially the mixed populations performed a homoacetic fermentation of cellulose in combination with the production of methane from hydrogen and CO2. The isolates were divided into three groups according to their fermentative activities in pure culture or in association with a hydrogenoxydizing methanogen. Group I included four strains belonging to the species C. thermocellum. The 35 strains of group II and III were weakly or non cellulolytic but actively fermented xylan, starch and various sugars. Except on cellulose, lactate was the major end product even in coculture with the methanogen. As far as the peculiar behaviour of the strains of group II and III concerned, we were unable to assign them to any previously described species of Clostridia.