We report on the isolation, purification and functional characterization of a soluble c-type cytochrome from light-grown cells of the purple phototroph Rhodoferax fermentans. This cytochrome is basic (pI = 8), has a molecular mass of 12 kDa, and is characterized by a midpoint reduction potential of +285 mV. Partial analysis of the N-terminus amino-acid sequence shows a high similarity with cytochromes of c8 type (formerly called Pseudomonas cytochrome c-551 type). Time-resolved spectrophotometric studies show that this cytochrome c8 reduces the tetraheme subunit of the photosynthetic reaction center, in a fast (sub-ms) and a slow (ms) phase. Competition experiments in the presence of both cytochrome c8 and high potential iron-sulfur protein (HiPIP), isolated from the same microorganism, show that cytochrome c8 oxidation is decreased upon addition of HiPIP. These observations suggest that cytochrome c8 and HiPIP might play alternative roles in the photosynthetic electron flow of Rhodoferax fermentans.