P/O ratios were measured in membrane particles obtained from cells of Micrococcus denitrificans, while growing on different carbon sources. The membrane particles obtained from cells growing actively on glucose, succinate, ethanol and propanol as the carbon and energy sources catalyzed oxidative phosphorylation and yielded respective P/O ratios of 1.4, 1.2, 0.8, and 0.5 with NADH, and 0.8, 0.6, 0.6, and 0.5 with succinate as the electron donors. Not such a difference in P/O ratio is observed in intact resting cells grown with different carbon sources. It is concluded that the influence of the carbon source is probably directed towards the efficiency of oxidative phosphorylation in membrane particles and not in the growing cells. For the aerobic carbon source-limited chemostat cultures the following maximum growth yields were determined: 40.2 and 34.2 for succinate and oxygen, 41.7 and 36.5 for malate and oxygen, 81.4 and 39.4 for mannitol and oxygen, and 77.8 and 43.4 for gluconate and oxygen respectively. With a mathematical model (de Kwaadsteniet et al., in press) the P/O ratio was valued at 1.4-1.7. YATP at mu=0.2 was valued at 8.7-10.9; YmaxATP at 9.6-13.2 and me at 0.6-4.5 for the most precise experiment (gluconate-limited). The calculation of these growth parameters has been discussed.