Selenomonas ruminantium is a nonsporeforming anaerobe that ferments carbohydrates primarily to lactate, propionate, acetate and CO2. H2 production by this species has not been previously reported. We found, however, that some strains produce trace amounts of H2 which can be detected by sensitive gas chromatographic procedures. H2 production is increased markedly, in some cases almost 100-fold, when the selenomonads are co-cultured with methane-producing bacteria. Growth of the methane-producing bacteria depends on H2 production by the selenomonads and the subsequent use of H2 for the reduction of CO2 to CH4. Although no free H2 accumulates in the mixed cultures, the amount of H2 formed by the selenomonads can be calculated from the amount of methane produced. These studies indicate that the conventional methods for measuring H2 production by pure cultures do not provide an adequate estimate of an organism's potential for forming H2 in an anaerobic ecosystem when H2 is rapidly used, e.g., for formation of CH4.