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Surface-water, groundwater , and water-quality data were compiled to describe and evaluate the water resources of Fremont County, Wyoming. These data are needed to plan for and manage the increased demands for water in the County. The study was conducted in cooperation with the Wyoming State Engineer. The average annual runoff varied for two of three regions that occur in the county. Average annual runoff ranged from 0.90 to 22 inches per year in the Mountainous Region and from 0.06 to 0.72 inch per year in the Plains Region. Available streamflow data are insufficient for computing average annual runoff in the High Desert Region. The Wind River Formation of Tertiary age has the most well development records of 157 inventoried wells were included in this report. The Wind River Formation is the most areally extensive water-bearing unit that occurs at land surface. The second most commonly developed geologic unit is the Quaternary alluvium and colluvium (49 wells). Wells and springs that were inventoried during this study that had large measured discharge (more than 300 gallons per minute) were the Arikaree Formation of Tertiary age, the Phosphoria Formation and related rocks of Permian age, the Tensleep Sandstone of Permian and Pennsylvanian age, the Madison Limestone of Mississippian age, and the Bighorn Dolomite of Ordovician age. Geologic units in Fremont County are recharged by one or a combination of the following sources: (1) infiltration of precipitation at the outcrop area, (2) infiltration of surface water, (3) infiltration of irrigation water, and (4) leakage from another geologic unit. In the Sweetwater Basin, the general direction of groundwater movement in the Arikaree aquifer is toward the Sweetwater River. In the Wind River Basin the general direction of groundwater movement in various water-bearing units is toward the Wind River. Ground water is discharged through pumped wells and is naturally discharged by springs and seeps, by evapotranspiration, and by discharge to streams, lakes, drains, and other geologic units. Prior to 1981, Riverton's municipal water supply was entirely from ground water. Water levels in the well field typically were deepest in August when demand for water was greatest. Since 1981, ground water is pumped only to supplement the surface-water treatment plant. Consequently, the water levels now are deepest in the winter and spring (January through May). Water levels in the Wind River Formation near the Riverton municipal well field also appeared to recover in 1983-85 after the plant began operating in 1981. Surface water supplies about 99 percent (592 million gallons per day in 1990) of the total offstream use in Fremont County. Irrigation is the largest offstream use of surface water. The largest use of ground water is for public supply. Total groundwater use in 1990 was 5.9 million gallons per day.