P/2e ratios were calculated from anaerobic chemostat cultures of Paracoccus denitrificans with nitrogenous oxides as electron acceptor. P/2e ratios were calculated, using the YmaxATP values determined for aerobic cultures. When succinate was the carbon and energy source the average P/2e values of the sulphate- and succinate-limited cultures with nitrate as electron acceptor were 0.5 and 0.7, respectively, and of the nitrite-limited culture 0.9. With gluconate as carbon and energy source and average P/2e values of the gluconate-limited with nitrate as electron acceptor and nitrate limited cultures were 0.9 and 1.1, respectively. leads to H+/O ratios measured in cells obtained from sulphate-, succinate, nitrite-, gluconate- and nitrate-limited cultures yielded respective average values of 3.4, 4.5, 3.5, 4.8 and 6.2 for endogenous substrates. From our data we conclude that sulphate- and nitrite- limitation causes the loss of site I phosphorylation. Nitrite has no influence on the maximum growth yield on ATP. We propose that metabolism in heterotrophically grown cells of Paracoccus dentrificans is regulated on the level of phosphorylation in the site I region of the electron transport chain.