The acetogen Clostridium thermoautotrophicum was cultivated under CO-dependent chemolithotrophic conditions. CO-dependent growth profiles and energetics indicated that supplemental CO2 was fundamental to efficient growth at the expense of CO. Overall product stoichiometry approximated 6.5CO --> CH3CO2H + 3.5CO2 + 0.6 cell C + 0.5 unrecovered C. Initial CO/CO2 ratios of 2 to 4 yielded optimal doubling times and cell yields. Maximal YCO values approximated 2.5 g of cell dry weight per mol of CO consumed; Y H2 , was considerably lower than Y CO Cross-transfer growth experiments and protein profiles indicated differential expression of genes between CO and methanol cultures.