Data from 23 magnetotelluric sites were used to determine electrical conductivities within the crust and upper mantle in Segenal (West Africa) along a profile about 600 km long. Data have been obtained in a variety of tectonic zones: the West African craton (stable since 1850¿250 m.y.), the West African mobile belt (with ages in the range of 350--650 m.y.), and the Senegal sedimentary basin. An earth model is derived from two-dimensional modeling. The model shows several heterogeneities extending deep in the lithosphere and, perhaps more deeply, in the asthenosphere, to a depth of about 460 km. At the stations in the western part of the basin the average resistivity is anomalously low (20--30 ohm m) down to about 10 km. The basin anomaly in the upper crust is assumed to mark an initial rift zone; it may reveal a pattern of crustal weakness in this area. Beneath this zone, the resistivity is in the range of 1000--3000 ohm m down to at least 300 km. In the eastern basin the resistivity is decreasing in the depth range of 20--30 km, and the top of the ultimate mantle conductor is at a depth of 300 km. At the sites in the marginal part of the West African craton, conductive layers can be recognized in the depth range of 30--40 and 80--100 km. The resistivity drops to 10 ohm m at a depth of about 460 km. The model in this region is compared with available geological and geophysical information, and an attempt is made to explain some of the observed discontinuities by plate tectonics. It is suggested that the major discontinuity which separates the basin and the mobile belt and extends to about 460 km depth could be interpreted as marking a Precambrian subduction slab dipping eastward. In the West African craton a low resistivity has been determined for a depth range of 130--150 km. The upper 130 km were found to have the rather high resistivities of 100--3000 ohm m. A sharp rise in conductivity occurs at a depth of about 460 km beneath the craton. In contrast, a surprising feature in the Kedougou region (in the craton) is that the major portion of the lower crust is conductive. The existence of a Birrimian geosynclinal pair in eastern Senegal consisting of a eugeosynclinal marine andesitic volcanic trough, particularly volcanics with ophiolite affinity, and a miogeosynclinal sedimentary sequence could characterize a modern plate tectonic phase dominately by widespread ocean opening and continental collision. The conductivive material in the lower crust might be explained by the buried ocean crust.