Native sulfur and associated calcite of Gulf Coast and Sicilian sulfur deposits may have a bacterial origin, as indicated by geological evidence as well as the isotopic compositions of these deposits. The δS 34 values of sulfides exhibit the characteristic variable enrichment of S 32 over that in the inferred sulfate source. The anaerobes may have reduced sulfate to hydrogen sulfide which was then oxidized exothermally and inorganically to native sulfur. Organic matter or petroleum constitutents presumably provided the energy source for the anaerobes. Field and laboratory data show that anaerobes produce isotopically heavier carbon dioxide and lighter methane than the associated organic matter which they consumed as an energy source. The cap-rock calcite associated with native sulfur in Gulf Coast salt domes is highly enriched in C 12 , suggesting that it formed from bacterial methane which was oxidized to carbon dioxide and then reacted with calcium ions derived from the sulfate. Limestone associated with the Sicilian native sulfur deposits is not as highly enriched in C 12 as the calcite in salt domes. Presumably, this limestone derived its carbonate primarily from oxidized methane enriched in C 12 and also from biogenically produced carbon dioxide enriched in C 13 .