Growth of thermophilic cellulase-utilizing bacteria in a vitimin-free growth medium is reported for both a previously described strain, Clostridium thermoclelum 31549, and now isolates HJA1 and HJA2. Formation of fermentation products with and without vitamins was similar for strains HJA1 and HJA2 as well as for the enrichment cultures from which these stains were derived. Strain HJA2 was maintained in continous culture on a vitamin-free mineral medium with Avicel as the carbon source for over a week. At a 38 h residence time, Avicel conversion was higher (81%) at pH 6.42 than at pH 6.97 (73%) or at 6.01 (58%). Ethanol and acetate were produced in significant amounts by strain JHA2 at all pH values tested (6.97, 6.42, 6.01). Lactic acid was the primary fermentation product at pH 6.97, but was not a significant product at both the lower values. Efforts to grow thermophilic, cellulose-utilizing bacteria at pH < 6.0 were unsuccessful for described strains, new isolates, and enrichment cultures.