There are two equally abundant high-potential iron sulfur protein (HiPIP) isozymes present in the purple sulfur bacterium Ectothiorhodospira vacuolata. We have determined the amino acid sequences, which contain 71 and 72 residues. The two HiPIPs can be aligned without any internal insertions or deletions and are 65% identical to one another. The E. vacuolata HiPIPs are most similar to the HiPIP isozymes from Ectothiorhodospira halophila (32-36% identity) and require at least one internal gap for alignment. Other HiPIPs require greater numbers of insertions and deletions for alignment with those of E. vacuolata and E. halophila, and the percentage similarities are slightly smaller (19-40% identity). The E. vacuolata HiPIP isozymes appear to be slightly closer to other species than are the E. halophila isozymes. The E. vacuolata and E. halophila HiPIPs also show slightly greater similarity to the five species of Chromatiaceae, which have been studied, and less similarity to the non-sulfur purple species. These results are in agreement with other studies, which indicate that the two purple sulfur bacterial families, Ectothiorhodospiraceae and Chromatiaceae, are more closely related to one another than to the Rhodospirillaceae.