Some Authors consider the Circeo Promontory an autochthonous structure, others regard it as a “Klippe” of Mesozoic limestone overlying Cenozoic flysch. From this geolectric study it appeared that under some ten meters of very resistive carbonate rocks lies a conductive formation characterized by a very low electric resistivity similar to the one of the flysch outcropping near by the Promontory. On the other hand it is possible to exclude what was suggested by some Authors i.e. that the substrate low resistivity is due to the influence of sea water flowing inside the carbonate, because the contact plane between the carbonate and the underlying formation lies over the sea level. It is therefore possible to conclude that Circeo Promontory is composed of allochthonous masses of Mesozoic carbonate rocks overlying Cenozoic flysch.