Groundwater samples from wells and springs, scattered over most of the Great Basin province, were collected and analyzed for their isotopic makeup. They were augmented by previously published isotopic data on groundwaters from southeast California and by several hundred unpublished isotopic analyses. The ratio of ²H (deuterium, D) to ¹H, in water samples from valleys in parts of California, Idaho, Nevada, Oregon, and Utah, are here compared with the winter, summer, and annual isotopic compositions of precipitation falling in or near the sampled areas. The main goal of this study was to identify basins where the groundwaters have isotopic compositions that are “lighter” (depleted in the heavier isotope, D) relative to modern winter precipitation. Where these basins do not adjoin substantially higher terrain, we consider those light groundwaters to be of Pleistocene age and thus more than 10,000 years old. Where the groundwater is 10 to 19‰ lighter than local winter precipitation, we consider it to be possibly an indication of Pleistocene water; where the δD makeup is >20‰ lighter, we consider it to be probably Pleistocene water. More than 80 sites underlain by waters of possible or probable Pleistocene age were identified.