The anaerobic cellulolytic rumen bacterium Ruminococcus flavefaciens normally produces succinic acid as a major fermentation product together with acetic and formic acids, H2, and CO2. When grown on cellulose and in the presence of the methanogenic rumen bacterium Methanobacterium ruminantium, acetate was the major fermentation product; succinate was formed in small amounts; little formate was detected; H2 did not accumulate; and large amounts of CH4 were formed. M. ruminantium depends for growth on the reduction of CO2 to CH4 by H2, which it can obtain directly or by producing H2 and CO2 from formate. In mixed culture, the methanobacterium utilized the H2 and possibly the formate produced by the ruminococcus and in so doing stimulated the flow of electrons generated during glycolysis by the ruminococcus toward H2 formation and away from formation of succinate. This type of interaction may be of significance in determining the flow of cellulose carbon to the normal rumen fermentation products.