The 61-residue amino acid sequence of Rhodospirillum tenue, strain 2761, high-redox-potential ferredoxin (HiPIP) is GTNAAMRKAFNYQDTAKNGKCSGCAQFVPGASPTAAGGCKVIPGDNEIAPGGYCDAFIVKK. It differs from that of R. tenue strain 3761 by 16 amino acid substitutions plus two single-residue deletions. This 26% sequence difference is similar to that observed among separate species of chromatiaceae such as Chromatium vinosum, C. gracile, and Thiocapsa roseopersicina, and is suprising because there are no distinguishing microbiological characteristics separating these two R. tenue strains. The most interesting amino acid substitution in R. tenue 2761 HiPIP is Gly for Asn 45 (C. vinosum numbering). Besides the four cysteines required to bind the four iron-four sulfur cluster, only Tyr 19, Asn 45, and Gly 75 are absolutely conserved in the nine previously determined HiPIP sequences. If HiPIP is used as a measure of divergence of species, then R. tenue and C. vinosum are the most distant purple bacteria examined. Quite the opposite conclusion follows based on the sequences of the cytochromes c'. It is suggested that this anomaly is more likely owing to a change in function for HiPIP with subsequently rapid evolutionary change than to a relatively recent transfer of the cytochrome c' gene between species.