We have determined the amino acid sequence of a ferredoxin from a photosynthetic green sulfur bacterium, Chlorobium thiosulfatophilum strain Tassajara. It contains 61 amino acid residues with 9 cysteines, and 8 of the 9 were located at positions corresponding to those in clostridial-type ferredoxins. Other structural features were closer to those of ferredoxins from another photosynthetic bacterium, C. limicola, than to those of non-photosynthetic bacteria. Compared with ferredoxin from Chromatium, a photosynthetic purple sulfur bacterium, all photosynthetic bacterial ferredoxins have a common region in the carboxyl-terminal half with several extra residues and a unique cysteine residue. We compared all the photosynthetic bacterial ferredoxins that have been sequenced and concluded that C. thiosulfatophilum ferredoxin is most closely related to C. limicola ferredoxin I.