Glucose and xylose are the principal monomeric units of plant carbohydrates. Butyrivibrio fibrisolvens, an important rumen bacterium, converted both sugars to formate, acetate, butyrate and lactate. More metabolites and less cell biomass was formed from xylose than from glucose. In cultures with both substrates, glucose was utilized preferentially. Growth on glucose was more rapid than on xylose. No phosphoketolase activity (EC 4.1.2.9) was detected in this strain. More carbohydrate and less protein was found in glucose-grown cells than in cells grown on xylose. Growth yields of protein were higher on xylose, in spite of the fact, that yields of cellular dry matter were higher on glucose.