Abstract The southern part of the Senegalo-Mauritanian basin is geologically featureless, and little is known about the division of the sedimentary sequence and regional geological structure because of the scarcity of deep drillholes. A magnetotelluric survey was, therefore, carried out in an attempt to obtain details concerning the electrical characteristics, thickness and structure of the Mesozoic-Cainozoic Senegalo-Mauritanian basin. From the surface down, the interpretation of magnetotelluric data shows several distinct electrical boundaries. The general trend shows a decrease in near-surface resistivity from east to west. The basin is characterized by water-saturated, unconsolidated, low-resistivity sediments, with a westward-sloping monoclinal style. The combination of borehole and geological data with a geoelectrical cross-section provides a simplified model of the geological-electrical units of the basin. This shows the distribution of strata above and below the base of the Senonian. Conductive material (10–30 ohm m) at depth in the central part of the profile may represent a northern extension of the Palaeozoic sediments of the Bove sequence, which lies immediately south of the magnetotelluric line. Significant electrical contrasts between the sediments and basement rocks allow mapping of the top of the high-resistivity basement, which is characterized by progressively deeper burial to the west.