Energy production in Gl. liquefaciens was studied by measuring oxidative phosphorylation in cell-free extracts and molar growth yield in growing cultures. P/O ratios were found very low and ranged from 0.09 for DPNH to 0.40 for glucose 6-phosphate, that is about 5% of the efficiency in animal mitochondria. The low P/O ratios truly represent the low efficiency of energy transfer in this organism, as in growing cultures the molar growth yield is equally low. From the P/O ratio and the molar growth yield for the oxidation from ethanol to acetate the yield in milligram dry cells/mole of ATP produced was calculated at 10 mg/mmole ATP. This value is in close agreement with that reported for other microorganisms. The low efficiency of energy transfer is partly explained by the presence of only one cytochrome in this organism. The absence of respiratory control (indicated by insensitivity to DNP) suggests that the oxidation is very loosely coupled to phosphorylation.