GIS-based predictive mapping to locate high-potential areas for prehistoric archaeological sites is becoming increasingly popular among archaeologists. Knowledge of the environmental variables influencing activities of original inhabitants is used to produce GIS layers representing the spatial distribution of these variables. The GIS layers are then analyzed to find locations where combinations of environmental variables match patterns observed in known sites. Presented is a study to locate high-potential areas for prehistoric sites in a largely unsurveyed area of 39,000 acres in the Upper Chesapeake Bay including details of the analysis process. The project used environmental data from over 500 known sites in other parts of the region and results corresponded well with known sites in the study area.