The study of the quality, temperature, and gas content of ground water and of the distribution of sulfate-reducing bacteria in the Lower Fars Formation of northern Iraq shows that minable sulfur deposits result from sulfate reduction and hydrocarbon oxidation in the presence of the sulfate-reducing bacteria Desulfovibrio desulfuricans. The maximum population of this bacteria was found where there is continuous supply of hydrocarbon gases, sulfate ions, and circulating water. In one such area the hot springs of Hammam Al-Alil, the water temperature reached 48 degrees C. Temperatures as high as 73 degrees C were found capable of supporting bacterial growth in the Mishraq area, where sulfur is being mined by the Frasch method.The richest sulfur ore was deposited in anticlinal structures containing an ample supply of hydrocarbon gases and a continuous supply of sulfate ions from the soluble gypsum and anhydrite in the Lower Fars Formation. In these anticlines, a continuous flow of ground water takes place in an open hydrologic system (water-table aquifer). The hydrogen sulfide which evolves and migrates from the lower parts of the structure is oxidized by oxygen in the ground water.