Elemental sulfur occurs widely distributed in two geologic environments: basins containing hydrocarbons and zones of Cenozoic volcanism. A classification is suggested for these deposits. Essentially all economically important elemental sulfur deposits being mined today are bioepigenetic replacements of anhydrite or gypsum. These occur in two geologic styles: in cap rocks over salt diapirs and in stratabound deposits. However, all of these deposits appear to share remarkably similar origins and characteristics. They are formed in evaporite basins where petroleum-bearing beds underlie anhydrite or gypsum and occur where joints or faults permit water, hydrocarbons, and bacteria to rise into the evaporites. The bacteria oxidize hydrocarbons to CO 2 , reduce sulfate ions to H 2 S, and alter gypsum to calcite. Data indicate that H 2 S is converted to polysulfides and these are oxidized to elemental sulfur by CO 2 in anaerobic environments. From two to four barrels of oil must be consumed per ton of sulfur, thus enormous amounts of hydrocarbons are required to form large deposits. These quantities are obtained by long-sustained artesian flushing by ground waters of areally extensive petroleum-bearing strata. A combination of unique structures form the plumbing systems required to concentrate artesian flow in the evaporites and thus localize the sulfur deposits.