Band-limited magnetotelluric measurements were collected at several sites across a wide area in the California Basin and Range and northern Mojave Desert from early 1988 until March 1989. Due to the large spacing between sites and the limited frequency range of the data, there are limits to what the data can resolve. However, a three-dimensional (3-D) magnetotelluric modeling study and sensitivity analysis of a subset of the data support the following conclusions. (1) Electrical currents from the Pacific Ocean are channelled across the western Mojave Desert and into the southern Basin and Range. (2) The 3-D geometry of the basins significantly influences the fit to the data mode aligned along the strike of the basins (transverse electric mode). (3) The lower crust in the California Basin and Range is more conductive than the lower crust under the Mojave Desert and Pacific Ocean. The resistivity values we modeled for the lower crust in the California Basin and Range, though inconsistent with mineral semiconduction, can be explained by electrical conduction through small amounts of saline fluids. ¿ American Geophysical Union 1996