An anaerobic, cellulose decomposing bacterium was isolated in pure culture from the ingesta of a bovine rumen. The organia was a small, curved, Gram negative rod that occurred singly or in chains. The isolated bacterium was identified as a member of the genus Butyrivibrio because of its morphology and because of its production of certain volatile fatty acids in a rumen fluid-glucose medium fermentation. The characteristic fermentation waa the production of a large amount of butyric acid and some lactic acid or succinic acid, and the lack of production of propionic acid. It was found that when rumen fluid was omitted from the fermentation medium the production of butyric acid decreased markedly. The organism was found to have a relatively narrow pH tolerance for the initiation of growth, pH 6.3 to 7.2, with an optimum of pH 6.9 to 7.0. Growth stopped at about pH 6.4, and continued fermentation decreased the pH to 6.0. The organism was cultured continuously on a medium which vu chemically defined except that vitamin-free casein hydrolysate was included, when the hydrolysate was replaced by a mixture of pure L-isomers of amino acids similar in composition to the hydrolysate, good growth occurred. However, when each amino acid was supplied at a uniform weight concentration, much weaker growth was obtained. Evidence was obtained indicating that aadno acids were inhibitory, although the reason for the mild inhibition was not disclosed. The following amino acids were found to be critical nutrients, but in some cases their status, whether essential or stimulatory, was found to be dependent on the composition of the medium: - cysteine, histidine, isoleucine. methionine, lysine, tyrosine, asparagine and leucine. A medium containing only these amino acids supported only very weak growth. Aspartic acid was found to be mildly inhibitory to growth, and glutamic acid was found to reverse the inhibition. A mixture of the common purine and pyrimidine bases, and a mixture of volatile fatty acids, were each found to be inhibitory to growth. Biotin, folic acid, and pyridoxal were essential vitamins, but the other eight B-vitamins examined were found to be neither stimulatory nor essential. Carbon dioxide was found to be required in relatively high concentration in order for growth to start in a medium devoid of rumen fluid. Purines, pyrimidines, cobalamine, and Tween-80 in a mixture were found not to replace the carbon dioxide. Rumen fluid was found to contain material very stimulatory for growth of the Butyrivibrio. The stimulatory material was found to be possibly two compounds or types of compounds, one a peptide or peptide like substance, and the other an anionic substance which probably was polynucleotide-like. / Ed. D.