The U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency, is studying the hydrogeology and water quality of aquifers in the vicinity of Belvidere, Boone County, Ill. This report presents available well-construction, hydrogeologic, and ground-water-quality data compiled into a geographic information system (GIS) data base for this study. General instructions on manipulation and suggested applications of the GIS data base are included. Three plates show the location and type of wells and distribution of ground-water-quality data within the study area. The data are intended to aid investigators in present and possible future hydrogeologic studies in the vicinity of Belvidere. The data base contains information for 725 water-supply and monitoring wells and lithologic borings. The sources of the data and methodology utilized to develop the data base are discussed. Well-construction and hydrogeologic data include well-identification label, latitude and longitude of the well locations, type of well, aquifer to which the well is open, total depth of the well, depth to water in the well, the screened or open interval, and owner information. Water-quality data are tabulated for concentrations of selected volatile organic compounds (VOC's) and semivolatile organic compounds, inorganic compounds, and total pesticide, and specific conductance. Data are provided on a floppy diskette accompanying the report. About 78 percent of the wells in the data base are water-supply wells, including eight municipal water-supply wells. About 40 percent of the wells are open to the glacial drift aquifer, and 53 percent are open to the Galena-Platteville aquifer. The remaining wells are open to multiple-bedrock aquifers. Of the 157 wells and borings with available water-quality data, 59 percent are open to the glacial drift aquifer, 24 percent are open to the Galena-Platteville aquifer, and 17 percent are open to multiple-bedrock aquifers. VOC's have been detected in six municipal and three domestic wells. Maximum contaminant levels of four VOC's have been exceeded periodically in four municipal wells.