Green sulfur bacteria (GSB) utilize various combinations of sulfide, elemental sulfur, thiosulfate, ferrous iron, and hydrogen for anaerobic photoautotrophic growth. Genome sequence data is currently available for 12 strains of GSB. We present here a genome-based survey of the distribution and phylogenies of genes involved in oxidation of sulfur compounds in these strains. Sulfide:quinone reductase, encoded by sqr, is the only known sulfur-oxidizing enzyme found in all strains. All sulfide-utilizing strains contain the dissimilatory sulfite reductase dsrABCEFHLNMKJOPT genes, which appear to be involved in elemental sulfur utilization. All thiosulfate-utilizing strains have an identical sox gene cluster (soxJXYZAKBW). The soxCD genes found in certain other thiosulfate-utilizing organisms like Paracoccus pantotrophus are absent from GSB. Genes encoding flavocytochrome c (fccAB), adenosine-5'-phosphosulfate reductase (aprAB), ATP-sulfurylase (sat), a homolog of heterodisulfide reductase (qmoABC), and other enzymes related to sulfur utilization are found in some, but not all sulfide-utilizing strains. Other than sqr, Chlorobium ferrooxidans, a Fe2+ -oxidizing organism that cannot grow on sulfide, has no genes obviously involved in oxidation of sulfur compounds. Instead, Chl. ferrooxidans possesses genes involved in assimilatory sulfate reduction (cysIHDNCG), a trait that is not found in most other GSB. Given the irregular distribution of certain enzymes (such as FccAB, AprAB, Sat, QmoABC) among GSB strains, it appears that different enzymes may produce the same sulfur oxidation phenotype in different strains. Finally, even though the GSB are closely related, sequence analyses show that the sulfur metabolism gene content in these bacteria is substantially influenced by gene duplication and elimination and by lateral gene transfer both within the GSB phylum and with prokaryotes from other phyla.