The cellulose-degrading species recently isolated from the human colon showed diverse ability to degrade and ferment cellulose. In the present study, the nature of the inter-relation existing between one H(2)-producing cellulolytic isolate (Ruminococcus sp. nov.) and one non-H(2)-producing cellulose-degrading species (Bacteroides sp. nov.) was investigated in vitro. Coculture experiments revealed synergism in cellulose degradation between these two cellulolytic species. An increase in total bacterial population was measured in the coculture, Bacteroides sp. being the predominant organism. As a result, a large decrease in H(2) production from cellulose fermentation was observed. Predominance of Bacteroides sp. might thus contribute to limit gas produced from fibre fermentation in the gut.