Bradyrhizobium japonicum utilizes cytochrome cbb3 oxidase encoded by the fixNOQP operon to support microaerobic respiration under free-living and symbiotic conditions. It has been previously shown that, under denitrifying conditions, inactivation of the cycA gene encoding cytochrome c550, the electron donor to the Cu-containing nitrite reductase, reduces cbb3 expression. In order to establish the role of c550 in electron transport to the cbb3 oxidase, in this work, we have analyzed cbb3 expression and activity in the cycA mutant grown under microaerobic or denitrifying conditions. Under denitrifying conditions, mutation of cycA had a negative effect on cytochrome c oxidase activity, heme c (FixP and FixO) and heme b cytochromes as well as expression of a fixP'–'lacZ fusion. Similarly, cbb3 oxidase was expressed very weakly in a napC mutant lacking the c-type cytochrome, which transfers electrons to the NapAB structural subunit of the periplasmic nitrate reductase. These results suggest that a change in the electron flow through the denitrification pathway may affect the cellular redox state, leading to alterations in cbb3 expression. In fact, levels of fixP'–'lacZ expression were largely dependent on the oxidized or reduced nature of the carbon source in the medium. Maximal expression observed in cells grown under denitrifying conditions with an oxidized carbon source required the regulatory protein RegR.