Acetomonas oxydans is not able to grow on ethanol because of the lack of enzymes of the tricarboxylic acid cycle. Ethanol is merely oxidized to acetic acid. However, it was shown that Am. oxydans can utilize the energy from the oxidation of ethanol to acetic acid for growth. In this respect alcohol can be replaced by lactate. P/O ratios were measured with cell-free extracts and the following substrates: ethanol, lactate, pyruvate, acetaldehyde, NADH2 and NADPH2. The P/O values were identical when the cells were grown on the same medium. Glucose grown cells gave a P/O ratio for ethanol or lactate of 0.08. But with glucose-ethanol grown cells P/O ratios of 0.28 were obtained. Ethanol can be replaced by lactate for cell cultivation and as a substrate for the oxidative phosphorylation. In each oxidation step, i.e. ethanol→acetaldehyde, lactate→pyruvate, and acetaldehyde→acetate, the same amount of ATP is produced per mole oxygen consumed when the cells were grown under comparable conditions.