A sulfur, carbon, and oxygen isotopic study of more than 300 samples has been made of the sandstone-type uranium deposits of the Gas Hills district, Wyoming. The deposits occur as rolls and blankets in the arkosic, lower Eocene Wind River formation located on the southern edge of the Wind River Basin.Pyrite cement associated with uranium mineralization (mean delta S 34 = --23ppm, + or - 12.3ppm) is isotopically similar to the acid soluble sulfides produced by anaerobic bacteria in nearby springs and is significantly lighter and more variable in delta S 34 composition than undoubted magmatic hydrothermal sulfides. Syngenetic-diagenetic pyrite nodules in barren siltstones have a delta S 34 composition of 3.7ppm, + or -10.6ppm. Isotopic analyses of hydrogen sulfide from neighboring oil fields, pyrite in granitic Precambrian cobbles, and groundwater sulfate from the overlying tuffaceous strata are consistent with, but do not prove, the "volcanic leach" hypothesis of origin for the sulfur of the epigenetic pyrite.The very light delta S 34 composition of pyrite associated with jordisite and coalified wood, and the enrichment of S 34 in selenium-bearing pyrite and groundwater sulfate of the host Wind River formation suggest limited production of hydrogen sulfide, resulting in isotopically heavy supergene sulfides. The extremely light isotopic composition of pyrite associated with jordisite on the leading edges of roll ore bodies is probably caused by oxidation and re-reduction of previously formed biogenic sulfides.Further evidence of a biogenic origin for these deposits is furnished by the carbon isotopic composition of epigenetic calcite cement (--22ppm, + or -4.0ppm) that is intimately associated with organic material. The similarity of the delta C 13 values of calcite associated with asphalt and the calcite associated with coalified wood implies that the anaerobes derived their energy from both. The heavier isotopic composition of most of the other epigenetic calcites suggests some contamination by sedimentary carbonate during calcite precipitation.The epigenetic calcite cements contain some of the lightest oxygen yet reported for carbonates (mean delta SMOW O 18 = 5.6ppm, + or -6.0ppm) and may indicate that the uranium deposits formed during Pleistocene time.This study shows that syngenetic-diagenetic and epigenetic deposits of bacterial origin have distinguishable sulfur and carbon isotopic characteristics. The extremely light delta S 34 composition of sandstone-type uranium deposits is believed to be primarily due to the reduction of fairly light sulfate by starved anaerobes in an environment that approximates a closed system. Further enrichment in S 32 occurs when these isotopically light sulfides are oxidized to light sulfate which is again reduced to pyrite during the formation of roll ore bodies.