The principal water-bearing rocks underlying Brown County, Wis., are thick sandstone units of Cambrian and Ordovician age. Other aquifers include limestone and dolomite of Ordovician age, dolomite of Silurian age, and sands and gravel of Pleistocene and Recent age. Underlying the water-bearing formations are crystalline rocks of pre-Cambrian age which contain little or no water. Ground water is the source of all public and most private and industrial supplies in. the county. Several of the large industries use large quantities of surface water also. Most of the water is pumped from wells that penetrate the Cambrian sandstones where the water occurs under artesian conditions. From 1886, when the first deep well was drilled, to 1949, the pumpage in the county increased to an average of about 5 million gallons a day (mgd) in 1939 and to about 10 mgd in 1949. The piezometric level, which was about 100 feet above the land surface in 1886, was about 300 feet below the land surface in 1949. About 200 feet of this decline took place after 1938. The water-level-measurement program begun in 1946 shows that yearly fluctuations of water levels in observation wells range from less than 1 foot to about 90 feet, the fluctuations being larger at the center of the heavily pumped area. The highest water levels occur in the winter or spring and the lowest in the summer near the end of the season of maximum withdrawal. Coefficients of transmissibility and storage for the sandstones were obtained by making controlled pumping tests at Green Bay and De Pere. The coefficients were verified by comparing computed water-level declines and rates of withdrawal with actual ones. The computed values were within I0 percent of the actual values. Probable declines of water levels by 1960 were computed, using the same coefficients of transmissibility and storage, and assuming three different conditions of pumping. The additional decline in water level will be 15 to 150 feet in the center of the pumped area, depending upon the amount of increased pumping and its distribution relative to the present pumped area and to the recharge area. The water from the sandstones is a hard calcium magnesium bicarbonate water. Further work is needed to determine whether there is danger of contamination by salt water which occurs down the dip in the same formations. It is concluded that the rate of withdrawal from the area can be increased to 15 mgd by 1960 without dangerously lowering water levels, provided that new wells are properly spaced. In order to avoid expressive lowering of water levels, it is recommended that new wells be located west of Green Bay toward the recharge area. A detailed study has not been made of shallow aquifers in the county. Further work should be done to evaluate the possibilities of auxiliary supplies from the limestone of the Platteville formation or from the Niagara dolomite. Conservation should be practice.4 by all users of ground water to avoid waste resulting in lower water levels and higher pumping costs.