Bradyrhizobium japonicum cytochrome c550, encoded by cycA, has been previously suggested to play a role in denitrification, the respiratory reduction of nitrate to dinitrogen. However, the exact role of this cytochrome in the denitrification process is unknown. This study shows that cytochrome c550 is involved in electron transfer to the copper-containing nitrite reductase of B. japonicum, as revealed by the inability of a cycA mutant strain to consume nitrite and, consequently, to grow under denitrifying conditions with nitrite as the electron acceptor. Mutation of cycA had no apparent effect on methylviologen-dependent nitrite reductase activity. However, succinate-dependent nitrite reduction was largely inhibited, suggesting that c550 is the in vivo electron donor to copper-containing nitrite reductase. In addition, this study demonstrates that a cytochrome c550 mutation has a negative effect on expression of the periplasmic nitrate reductase. This phenotype can be rescued by extending the growth period of the cells. A model is proposed whereby a mutation in cycA reduces expression of the cbb3-type oxidase, affecting oxygen consumption rate by the cells and consequently preventing maximal expression of the periplasmic nitrate reductase during the first days of the growth period.