Magnetotelluric measurements made on the Colorado Plateau near Farmington, New Mexico, characterize the province as consisting of conductive surficial sediments underlain by a more resistive crystalline crust down to about 28 km. Beneath this zone the average resistivity is anomalously low (13 ohm m) down to at least 130 km. On the other hand, seismic evidence indicates a crustal thickness of approximately 40 km for the interior of the plateau, with lower than normal velocities in the upper mantle. The two sets of measurements are compatible providing an electrical transition zone is present in the lower crust separating a highly resistive upper crust from more conductive mantle material below 40-km depth. Constraints on the allowable upper mantle resistivity values, when combined with the theoretical analysis of Shankland and Waff on the effects of partial melt concentrations, suggest that the bulk resistivity of the mantle is being influenced by the presence of a small degree of partial melt (a few percent by volume). Temperature estimates associated with the allowable resistivity variations lie within the range 1000 ¿--1200 ¿C at 50-km depth and yield a geothermal gradient over the same depth interval of 3 ¿--4 ¿C/km or less. A comparison with electrical data that are available in the surrounding tectonic provinces indicates that the upper mantle beneath the Colorado Plateau has resistivities at least as low as those for the Basin and Range, Rio Grande rift, and Great Plains and may in fact be substantially more conducting than any of these surrounding tectonic provinces.