1. The electron paramagnetic resonance spectra at 15 K of reduced membrane particles of Paracoccus denitrificans exhibit resonance signals with g values, line shapes and temperature profile which are similar to the signals of the iron-sulfur centers observed in the NADH-ubiquinone segment of mitochondrial respiratory chains. These iron-sulfur centers are reducible with NADH, NADPH as well as chemically with dithionite. 2. Sulphate-limited growth of Paracoccus denitrificans results in the loss of an electron paramagnetic resonance signal (gz approximately 2.05, gy approximately gx approximately 1.92) which has properties similar to those of iron-sulfur center 2 of the NADH dehydrogenase of mitochondrial origin. The loss of this signal is accompanied by a decrease in the NADH oxidase and NADH ferricyanide oxidoreductase activities to respectively 30 and 40% of the values found for succinate-limited growth conditions. In addition respiration in membrane particles from sulphate-limited cells loses its sensitivity to rotenone. 3. Since sulphate-limited growth of Paracoccus denitrificans induces loss of site I phosphorylation [Arch. Microbiol. (1977) 112, 25-34] these observations suggest a close correlation between site I phosphorylation, rotenone-sensitivity and the presence of an electron paramagnetic resonance signal with gz approximately 2.05 and gy approximately gx approximately 1.92.