Crustal structure profiles relative to the Aegean fore arc area are modeled according to geoid undulations and free air gravity anomalies, taking into account the subducted slab. The models involve three layers with fixed density but a variable geometry for their boundaries. The first profile, drawn across the western (convergent) branch of the arc, exhibits a large amount of subducted crustal material, and a local trough in the Moho under the so-called Mediterranean Ridge, superimposed on a long-wavelength flexure increasing toward the trench. The Moho topography suggests lithosphere with no rigidity below the Mediterranean Ridge, while the flexure associated with the subduction implies an elastic thickness of 80 km. This discrepancy leads us to assume the presence of an additional load acting downward below the Mediterranean Ridge. On the second profile, drawn across the eastern (strike-slip) branch, there is a deep trough in the Moho, far from the topographic trench. This Moho deepening represents a large isostatic unbalance and requires an active compensation mechanism. Thus both profiles, and especially the eastern one where the strike-slip dominant motion does not allow a proper subduction, require a downward force acting seaward of the arc. On the latter branch, we propose that gravitational instability induces the growth of a cold bulb at the base of the old and dense African lithosphere. It is noted that, through viscous coupling with the top of the lithosphere, a 115-km bulb can produce an important collapse of the Moho and that the resulting gravitational anomalies are comparable to the observations. The growth of such a cold lithospheric bulb may be the first stage of a southward jump of the Aegean arc. ¿ American Geophysical Union 1986