The capability of high potential iron-sulfur proteins (HiPIPs) and soluble cytochromes to shuttle electrons between the bc1 complex and the terminal oxidase in aerobically grown cells of Rhodoferax fermentans and Rhodospirillum salinarum, two facultative phototrophs, was evaluated. In Rs. salinarum, HiPIP and a c-type cytochrome (alpha-band at 550 nm, Em,7=+290 mV) are both involved in the electron transfer step from the bc1 complex to the terminal oxidase. Kinetic studies indicate that cytochrome c550 is more efficient than HiPIP in oxidizing the bc1 complex, and that HiPIP is a more efficient reductant of the terminal oxidase as compared to cytochrome c550. Rs. salinarum cells contain an additional c-type cytochrome (asymmetric alpha-band at 556 nm, Em,7=+180 mV) which is able to reduce the terminal oxidase, but unable to oxidize the bc1 complex. c-type cytochromes could not be isolated from Rf. fermentans, in which HiPIP, the most abundant soluble electron carrier, is reduced by the bc1 complex (zero-order kinetics) and oxidized by the terminal oxidase (first-order kinetics), respectively. These data, taken together, indicate for the first time that HiPIPs play a significant role in bacterial respiratory electron transfer.