A magnetotelluric sounding is carried out in the period band ranging from 15 to 10, 000sec on the West-African craton in the Upper Volta Republic. The site is located geologically on the Birrimian formations (2, 000m.y.). Bi-dimensional modelling suggests a conductive layer in the upper mantle at a depth of 130km. We have to assume a layer of high mantle resistivity, extending from 150 to at least 465km depth. An ultimate reduction to 10ohm-m occurs at 465km depth. Resistivity models for the West African craton are presented (Senegal-Upper Volta-Niger). They imply a general reduction in mantle resistivity between 80 and 130km. With these models we can find no crust-upper mantle transition from the viewpoint of resistivity. The upper mantle resistivity of 3, 000ohm-m may be taken as representative of the resistivity for the African Shield.