The fermentative metabolism of Acetobacterium sp. grown on methanol-formate in continuous culture is described. The reaction stoichiometry of methanol-formate, including cells, were as follows: CH3OH + 1.13HCOOH --> 0.87CH(3)COOH + 0.47 cell C. Formate enhanced growth yields by approximately 60% compared with methanol-CO2-grown cultures. Comparison of yields on methanol-formate allowed calculation of an energy yield of 1.3 mol ATP per mol acetate formed during homoacetate fermentation. The magnitudes of Y-EG,the theoretical maximum yield of Y-E, and m, the maintenance coefficient, were determined by growing the organism in methanol-formate and resulted in 16.5 g cell (mol methanol catabolized)(-1) and 0.674 mmol methanol catabolized (g cell)(-1) h(-1), respectively. It is concluded that formate might replace CO2 as a source of carboxyl donor.