The kinetics and energetics of Acetobacterium sp. for growth and vitamin B-12 production were analyzed in chemostat cultures on a methanol-CO2 medium under various pHs, methanol concentrations and dilution rates. The optimal pH was between 6.7 and 6.9. The maximum volumetric rates of cell, acetate and vitamin B-12 production and methanol consumption were 0.06 g.l(-1).h(-1) for cell, 5.7 mmol.l(-1).h(-1) for acetate, 0.27 mg.l(-1).h(-1) for vitamin B-12 and 10 mmol.l(-1).h(-1) for methanol. The specific production rates of acetate and vitamin B-12 were associated with the specific growth rate. Analysis of energetics revealed that the true growth yield, Y-EG, was 8.14 g cell.(mol methanol catabolized)(-1) and the maintenance coefficient, m, was 0.479 mmol methanol catabolized.(g cell)(-1).h(-1). Based on the mass balances with respect to carbon and electron recovery for methanol consumed, the following reaction stoichiometry was obtained : 4CH(3)OH+ 2CO(2)-->2.34CH(3)COOH + 0.86 cell C. The conversion ratio of methanol to cell was about 16%. Finally, we offer a simple explanation for the bioenergetics of this bacterium, suggesting that Y-ATP can be calculated directly from the values of Y-X/P.