Resting cells of Clostridium sticklandii fermented l-ornithine as a single substrate by a coupled oxidation-reduction with proline as the electron acceptor. The products of the fermentation of ornithine alone were ammonia, alanine, acetate, and δ-aminovalerate, in order of concentration. Traces of CO₂, butyrate, and proline were also found. When an equimolar amount of proline was added along with ornithine, very little δ-aminovalerate was produced from the ornithine, but essentially all of the proline was reduced to this compound. The ratios of the other primary products were changed by the addition of proline. The primary products from ornithine fermented in the presence of proline were acetate, ammonia, alanine, and CO₂, in order of concentration. Studies with dl-ornithine-1-¹⁴C, dl-ornithine-2-¹⁴C, and dl-ornithine-5-¹⁴C demonstrated that the primary cleavage of this amino acid occurred between carbons 3 and 4. A high percentage of the isotope from carbons 1 and 2 was found in alanine, and most of that from carbon 5 was found in volatile acid. The CO₂ formed was derived from the carboxyl carbon. All of the radioactivity from the fermentation of dl-alanine-1-¹⁴C was found in ¹⁴CO₂. The alanine from ornithine was oxidized by d-amino acid oxidase to the same extent as dl-alanine, indicating that it was dl-α-alanine. Preliminary experiments with cell extracts indicated proline is an intermediate in the reduction of ornithine to δ-aminovaleric acid.