Magnetotelluric data have been used in southern Mauritania to study crustal structures related to the Senegal-Mauritania basin. Observations were made at 10 sites along a 225 km northwest-southeast profile, beginning near the Atlantic shoreline and ending near the western border of the Mauritanides orogenic belt. The determinant parameters of the magnetotelluric impedance tensor were interpreted to obtain layered models beneath each of the sites. For sites near the coast, one-dimensional modeling of the data was followed by two-dimensional modeling. The models produced indicate three important layers in the electrical stratigraphy. In the western part of the traverse, unexpectedly low resistivities of about 3 ohm ⋅ metres (Ω⋅m) occur in the Mesozoic formations, which rapidly thicken seaward. Beneath the sedimentary sequences, the basement rocks have fairly uniform resistivities (1000-1500 Ω⋅m). Conductive layers were detected within the crustal section in the eastern part of the basin. In the depth range of 13 to 17 km, resistivities decrease to less than 20 Ω⋅m. This 2-5-km-thick zone dips gently eastward, suggesting the presence of a free fluid phase. By combining the information derived from the onshore geoelectrical model and the offshore seismic and deep-well data, a schematic geologic cross section through the Mauritania continental margin is proposed.