The complete amino acid sequence of Rhodoferax fermentans high-potential iron-sulfur protein (Hipip), which is known to be an efficient electron donor to the photosynthetic reaction center, has been determined using both N-terminal and C-terminal analyses. The sequence contains 75 residues, with 11 positive charges, 10 negative charges, and one histidine residue. The molecular mass of apo-Hipip, determined by electrospray ionization mass spectrometry, is 7849.64 Da. Multiple sequence alignment, based both on primary and tertiary structure information, reveals conservation of Tyr19 and Gly75 (Chromatium vinosum numbering) in addition to the four [Fe4S4]-bound cysteines. The Hipip from Rf. fermentans is most similar (57% similarity) to the Hipip from Rubrivivax gelatinosus, a photosynthetic bacterium belonging to the beta-1 subgroup of the proteobacteria.