Maps showing variations in the resistivity of the water in the 3 Lower Cretaceous sandstones in the Denver basin have been made by contouring determinations of resistivity based on the self-potential curves of electrical logs. The average error in each resistivity determination made by this method appears to be about 50%. There are some general similarities in the resistivity patterns of the 3 reservoir waters, but these resemblances might have occurred by chance. In most of the region studied, the resistivities of the formation waters are below 0.8 ohm-meter. In Cheyenne and Morrill counties, Nebraska, there is a sharp resistivity low with resistivities below 0.1 ohm-meter in all 3 sands. Toward the NE. and S. the resistivities are above 1.5 ohm-meters. The oil and gas in the Denver basin tend to occur in association with the more concentrated reservoir waters. The fact that the resistivity low in Cheyenne and Morrill counties, Nebraska, occurs in the same locality in all 3 sands indicates that there has been no appreciable hydrodynamic flow in any of the 3 sands in this area since the formation of the low. This resistivity low might possibly have been produced by highly saline waters rising from Paleozoic rocks through open fractures. The rather low resistivities of the reservoir waters in the 3 sands is in general unfavorable to the theory that the waters now in these aquifers have been derived from the outcrops by artesian flow. Accurate measurements of fluid pressure in several hundred wells in the Denver basin might determine definitely whether hydrodynamic flow has occurred in the Lower Cretaceous aquifers. Reliable pressure data could be obtained from drill-stem tests, but would apparently involve several months' work by a petroleum engineer experienced in this field.