was grown in continuous culture at different dilution rates under either methanol or oxygen limitation. Methanol dehydrogenase and cytochrome oxidase were maximally repressed in oxygen-limited cultures and also in methanol-limited cultures at dilution rates close to mU, whereas cytochrome oxidase was maximally repressed in methanol-limited cultures and also in oxygen-limited cultures at dilution rates close to μ. The observed changes in the activity and concentration of methanol dehydrogenase were accompanied by approximately parallel changes in the whole cell respiration rate with ethanol, formaldehyde or acetaldehyde, but not with methanol. These and other results suggest that whole cell methanol oxidase activity reflects respiration from both methanol and its oxidation product, formaldehyde, and is regulated via repression–derepression of methanol dehydrogenase and the cytochrome oxidases. The methanol oxidase system is thus precisely tuned to the concentration of methanol or oxygen in the culture such that the latter is able to maintain a rate of respiration that satisfies the energy demands of the imposed growth rate.