The principal water-bearing rocks underlying Fond du Lac County, Wis., are sandstones of Cambrian and Ordovician age and dolomite of Silurian age. Other aquifers include dolomite of Ordovician age and sand. and gravel of Quaternary age. Crystalline rocks of Precambrian age, which underlie all the water-bearing formations, form a practically impermeable basement complex and yield little or no water to wells. Ground water is the source of all public and most private and industrial water supplies in the county. The municipalities and industries obtain water chiefly from wells that penetrate the sandstones of Cambrian and Ordorician age. The Platteville formation and Galena dolomite of Ordovician age and the Niagara dolomite of Silurian age supply water to most domestic and stock wells and to a few industrial wells. Several buried valleys in the bedrock surface contain water-bearing deposits of sand and gravel. The source of the ground water in Fond du Lac County is local precipitation. Recharge to the water-bearing beds occurs in most of the county but is greatest where the bedrock formations are near the surface. Ground water is discharged by seeps and springs, by evaporation and transpiration, and by wells. Ground-water levels in wells fluctuate in response to recharge and to natural discharge and pumping. In areas not affected by pumping, water levels generally decline through the summer months because of natural discharge and lack of recharge, recover slightly in the fall after the first killing frost, decline during the winter, and recover in the spring when recharge is greatest. In areas of heavy pumping, the water levels are lowest in late summer and highest in late winter. Water levels in wells in the Fond du Lac area were about 5 to 50 feet above the land surface in 1885, but they had declined to as low as 185 feet below the land surface by 1957. Coefficients of transmissibility and storage of the sandstones of Cambrian and Ordovician age were determined by making controlled aquifer tests at Fond du Lac. The coefficients were verified by comparing computed water-level declines with actual declines. The computed values were within about 30 percent of the actual values, a reasonable agreement for coefficients of this type. Probable declines of water levels by 1966 were computed, using the same coefficients of transmissibility and storage. If the distribution of wells and the rate of pumping remain the same in 1957-66 as they were in 1956, the water levels will decline about 5 feet more by 1966. If, however, the distribution of pumped wells remains the same but the pumping by the city of Fond du Lac increases at a uniform rate from the 3 mgd (million gallons per day) pumped in 1956 to 5 mgd in 1966, the water levels in 1966 will be at least 60 feet below those of 1956. Dispersal of wells to the northwest toward the recharge area would reduce the water-level declines. The results of pumping tests, of test holes tapping the Niagara dolomite indicate that wells producing at least 200 gpm (gallons per minute) could be developed east of the Niagara escarpment. The ground water in Fond do Lac County is, in general, a hard calcium and magnesium bicarbonate water, which contains excessive iron in some areas.