The rate and extent of xylan utilization and the specific activities of extracellular polysaccharide-degrading enzymes formed by Ruminococcus flavefaciens FD1 were increased by cocultivation with Methanobrevibacter smithii PS. As a consequence of interspecies hydrogen transfer interactions, the fermentation became acetogenic; methane, not hydrogen, was formed, less succinate was produced, and formate did not accumulate in the coculture. Accumulation of xylobiose and xylose released during xylanolysis was transient in the methanogenic coculture. The interaction of R. flavefaciens and the hydrogen-utilizing acetogen Acetitomaculum ruminis also resulted in an acetogenic fermentation, higher polysaccharolytic enzyme activities, and increased xylan utilization; the effects of cocultivation of R. flavefaciens with Eubacterium limosum were not so pronounced.