A new taxonomy of the green sulfur bacteria is proposed, based on phylogenetic relationships determined using the sequences of the independent 16S rRNA and fmo (Fenna-Matthews-Olson protein) genes, and supported by the DNA G + C content and sequence signatures. Comparison of the traditional classification system for these bacteria with their phylogenetic relationship yielded a confusing picture, because properties used for classification (such as cell morphology, photosynthetic pigments and substrate utilization) do not concur with their phylogeny. Using the genetic information available, strains and species assigned to the genera Chlorobium, Pelodictyon and Prosthecochloris are considered, and the following changes are proposed. Pelodictyon luteolum is transferred to the genus Chlorobium as Chlorobium luteolum comb. nov. Pelodictyon clathratiforme and Pelodictyon phaeoclathratiforme are transferred to the genus Chlorobium and combined into one species, Chlorobium clathratiforme comb. nov. The name Pelodictyon will become a synonym of Chlorobium. Strains known as Chlorobium limicola subsp. thiosulfatophilum that have a low DNA G + C content (52-52.5 mol%) are treated as strains of Chlorobium limicola; those with a high DNA G + C content (58.1 mol%) are transferred to Chlorobaculum gen. nov., as Chlorobaculum thiosulfatiphilum sp. nov. Chlorobium tepidum is transferred to Chlorobaculum tepidum comb. nov., and defined as the type species of the genus Chlorobaculum. Strains assigned to Chlorobium phaeobacteroides, but phylogenetically distant from the type strain of this species, are assigned to Chlorobium limicola and to Chlorobaculum limnaeum sp. nov. Strains known as Chlorobium vibrioforme subsp. thiosulfatophilum are transferred to Chlorobaculum parvum sp. nov. Chlorobium chlorovibrioides is transferred to 'Chlorobaculum chlorovibrioides' comb. nov. The type strain of Chlorobium vibrioforme is phylogenetically related to Prosthecochloris, and is therefore transferred to this genus as Prosthecochloris vibrioformis comb. nov. Consequently, the name Chlorobium vibrioforme will become a synonym of Prosthecochloris vibrioformis, and other strains that were assigned to this species are now considered to belong to Chlorobium luteolum, Chlorobium phaeovibrioides and 'Chlorobaculum chlorovibrioides', according to their phylogenetic relatedness.