The electron-transfer pathway in the NADH-cytochrome b region of the respiratory chain in membrane fragments obtained from aerobically grown cells of Micrococcus denitrificans was found to consist of two types of sulfhydryl groups, a rotenone-sensitive site and coenzyme Q10, as in the same region in mammalian mitochondria. The sites of interaction of various acceptors with the NADH chain were investigated, and it was concluded that coenzyme Q0 was reduced at the NADH side of the flavin, the coenzyme Q10 site, and the cytochrome region. Significant phosphorylation was detected in the oxidation of NADH by coenzyme Q0. This phosphorylation was, however, only partly inhibited by antimycin A, owing to incomplete sensitivity of the respiratory chain to this antibiotic. The sensitivity to antimycin A could be greatly increased by treatment of the membrane fragments with phospholipase A [EC 3. 1. 1. 4]. Using this phospholipase-treated preparation, it was possible to confirm that phosphorylation at site I (NADH dehydrogenase region) was coupled only to the rotenone-sensitive reduction of coenzyme Q0 by NADH.