Prevotella ruminicola is a fibrolytic rumen bacterium that degrades complex carbohydrates and ferments the resulting hexoses and pentoses. Cultures of strain AR29 grew more rapidly on glucose than on xylose, but produced almost the same metabolite profiles on both carbon sources. Glucose in glucose-plus-xylose medium was used preferentially. The production of cell dry matter, growth yields of dry matter and protein, and cell composition (cell carbohydrate and protein) were not significantly different in glucose- and xylose-grown cultures. High aldolase (EC 4.1.2.13) activity was found both in glucose- and xylose-grown cells of the strain AR29. A very low phosphoketolase (EC 4.1.2.9) activity was detected in cells cultured on xylose. In contrast, enzymes of the EntnerDoudoroff pathway (6-phosphogluconate dehydrase and 2-keto-3-deoxy-6-phosphogluconate aldolase) could not be evidenced.