Fermentation of cellulose in the rumen occurs through the interactions of many microbial species. The initial degradation of cellulose is caused by cellulase-producing organisms. The soluble hydrolysis products are used by both cellulolytic and noncellulolytic organisms to produce acetate, propionate, and butyrate and the important intermediates H2 and succinate. Interactions between species are necessary for the decarboxylation of succinate to propionate. H2 is used by methanogenic bacteria to reduce CO2 to CH4. The removal of H2 by methanogenesis increases the production of acetate from carbohydrates by several important cellulose- and carbohydrate-fermenting microbial species. Monensin and lasalocid appear to alter the overall fermentation by selecting for populations that produce relatively larger amounts of propionate and against populations that produce relatively larger amounts of acetate and H2. Cellulose fermentation in the human large intestine is compared with fermentation in the rumen.