Abstract A study of the crust beneath the Madagascar Ridge and the submarine Crozet Plateau has established clear similarities between these two structures, located in symmetrical positions with respect to the Southwest Indian Ridge. Seismic refraction interpretation confirms the division of the Madagascar Ridge in a northern and a southern domain along latitude 31°S. No argument in favour of a continental nature of the northern domain can be put forward: this domain would rather correspond to a strongly anomalous oceanic crust. The deep structure of the southern domain ressembles that of the submarine Crozet Plateau; seismic velocities beneath these two structures are close to those encountered in the oceanic crust. The southern domain of the Madagascar Ridge and the submarine Crozet Plateau probably have a common origin and were later separated since the early Paleocene by spreading from the Southwest Indian Ridge. Local isostatic equilibrium of the northern domain of the Madagascar Ridge, achieved by crustal thickening, is confirmed by gravity models the depth to the Moho being close to that deduced from seismic refraction results. Gravity also confirms that the southern domain is unbalanced with respect to the northern one, and to the adjacent oceanic basins.