The metabolism of an impure culture of an aerobic cellulose decomposing bacterium, Vibrio perimastix, was studied. The products of cellulose decomposition included carbon dioxide, a pigment resembling riboflavin, a bacterial polysaccharide, and traces of acid. Carbon dioxide was found to be essential for the decomposition of cellulose and could not be replaced by calcium carbonate. Increasing the carbon dioxide content of the air above 1.2% retarded growth on a glucose medium.Respiration studies were carried out using cellulose, glucose, and cellobiose as substrates, and the possible role of the latter two as intermediates in cellulose decomposition was investigated. Glucose was produced from cellulose when toluene was added to cultures during active decomposition; evidence is presented that a dialyzable factor produced by the bacteria is essential for glucose formation. Phosphorylation inhibitors prevented growth of the bacteria on cellulose, glucose, and cellobiose, inhibited respiration of active cellulose cultures, and retarded the production of glucose by toluene-treated cultures. Cellulose treated with alkali to increase the proportion of amorphous to crystalline cellulose was more rapidly decomposed than untreated cellulose.