This report documents the geographic information system map layers and data files generated for the Santa Clara-Calleguas Basin, Ventura County, as part of a Regional Aquifer- System Analysis of southern California from 1989 to 1995. Thirty-six map layers and four data files are maintained in this geographic information system data base. The map layers cover the Santa Clara-Calleguas drainage basin and are stored in a common map projection. Attributes of the map layers and data files are described and referenced. The map layers are grouped by geography, geology, and hydrology.