The thermophilic Clostridium P2 was isolated from a semi-continuously fed reactor with high ammonium concentration. This bacterium formed substantial amounts of L-alanine as a major fermentation product from glucose, fructose and mannose. Low amounts of acetate, butyrate, carbon dioxide and hydrogen were also formed. A high partial pressure of hydrogen inhibited the degradation of the monosaccharides, whereas hydrogen removal, in the form of methanogenesis was found to be stimulatory. However, the amount of alanine produced per mole of hexose degraded did not change. Hexose degradation and alanine production were favoured by high ammonium concentrations. Nuclear magnetic resonance spectroscopy studies provided strong evidence that an active Embden-Meyerhof-Parnas pathway existed and that alanine was produced via an amination of pyruvate.