A high-resolution geoid map of the Madagascar and Mascarene basins is constructed from GEOS 3 and Seasat satellite altimetric data with an accuracy of 0.7 m. Large positive anomalies are due to topographic highs and their regional compensation. The deflection of the lithosphere under volcanic loads and its effect on the geoid are modeled for the Southern Mascarene Plateau, Mauritius, Reunion, and Rodriguez islands. Assuming a purely elastic lithospheric model, the effective flexural rigidity is estimated for the four zones. Rigidity varies from 1¿10+21 to 2¿10+23 N m, increasing from north to south. The highest rigidity occurs under Reunion Island and suggests a recent origin for the volcanism (3--5 Ma). The Southern Mascarene Plateau shows the lowest rigidity, suggesting a near-ridge origin; Rodriguez Island shows the same trend but in another tectonic framework. The topographic and geoid swell is studied as an effect of a thermal mantle plume. Using a conductive model of lithospheric heating, we confirm the assumption of ''hotspot migration'' for the origin of the Southern Mascarene Plateau and the Mascarene Islands. ¿ American Geophysical Union 1988