It is known that the electrical conductivity of the Earth's mantle increases to about 1 S/m at depths between 500 and 1000 km. This increase could equally well be ascribed to thermally activated conduction or to enhanced conductivity in high pressure mineral phases. Thermal activation would produce a smooth conductivity profile while conductivity associated with abrupt phase changes would also vary abruptly. Unfortunately, geomagnetic data alone cannot distinguish between these two models. However, under the assumption that the variation is indeed abrupt, we seek the best estimate for the depth to a conductivity jump. A peak in the geomagnetic spectrum around a period of 27 days produces an electromagnetic response at this period with least uncertainty and least bias associated with breakdown of an assumed P01 source-field geometry. Fitting such data using a simple two-parameter model of a buried conducting sphere, we estimate a conductivity of 1.18¿0.10 S/m at a depth of 650¿20 km. The coincidence of this result with estimated depths to the 660 km seismic discontinuity provides independent support for the hypothesis that the observed abrupt change in the elasticity of the mantle is also accompanied by an equally abrupt change in the electrical conductivity. Both physical properties are presumably associated with a mineral transition from an olivine-dominated upper mantle composition to perovskite/w¿stite assemblage. ¿ American Geophysical Union 1996