The interaction between homoacetogens and methanogens in lake sediments was investigated using hydrogen consumption as an indicator. Sediments samples were obtained from Lake Izunuma, Miyagi prefecture, Japan, a wintering place for migratory birds from Siberia. A batch experiment using H-2/CO2 as a substrate was conducted to determine the acetate generation and methane production potential of the sediments. Incubation for 4 d at 37 degrees C gave the following stoichiometric equation: 88H(2)+ 39HCO(3)(-) + 22H(+) --> 17CH(3)COO(-) + 5CH(4) + 83H(2)O. The activities, upsilon(m), of hydrogen-utilizing homoacetogens and methanogens respectively ranged from 3.2 to 48 and from 1.8 to 3.2 mgCOD.gVSS(-1).h(-1), The population of hydrogen-utilizing homoacetogens was determined to be 2.6 x 10(8) MPN.gVSS(-1), which was approximately two orders of magnitude higher than that of hydrogen-utilizing methanogens. The results suggest that homoacetogens in the sediments functioned not only as hydrogen consumers but also as major degraders of organic matter, forming acetate as the major reduction product.