A geomagnetic survey for the vertical component of the earth's field has been made in the region of lakes Vico and Bracciano, i. e., in the areas interested by the Vican and Sabatian volcanic structures, in view of a more detailed study of the magmatic masses that have alimented these structures. Interpretations of the results have led to following conclusion: a) a discontinuity exists between tlie magmatic mass that has alimented the Vican and Sabatian volcanism and the magmatic masses responsible for the volcanism in the area of lake Bolsena and of the Colli Albani; b) this magmatic mass may be assimilated to a rougly ellipsoidal body with the central point at approximately 7 to 10 km depth and the upper level very near to surface, and with lateral dimensions of the order of about 12-15 km, presenting upward protuberances that correspond to the crater of the Vico lake and to the area on the northwest of the Bracciano lake basin; c) there exist a lateral apophysis, connected with the main mass, which interests the craters of I'olline, Martignano, Baccano, and Sacrofano; it consists of a kind of plate of minor thickness, extending eastward at a depth of about 1-2 km, and roughly 5 km large.