Refraction data from the Agulhas Basin, south of southern Africa, show a crustal structure compatible with deep-water marine stations. Moho is about 10 km deep, and the crust is believed to be of oceanic origin. On the Agulhas Plateau, a basement layer having a seismic velocity of 4.84 km/s overlies the main crustal layer with velocity 6.72 km/s. This structure is not continental but resembles that of certain volcanic features in the Indian and Pacific Oceans, such as the Chagos-Laccadive and Hawaiian Ridges. The Agulhas Plateau is interpreted to be of oceanic origin. The plateau can be divided into two physiographic provinces. The southern province is characterized by a smooth basement overlain by relatively undisturbed sediment 0.5 to 1.0 km thick. In the northern province the basement topography is rough. The origin of the relief is not clear, but several possible models are suggested. The influence of bottom currents is marked in this region, and the sediments are more disturbed. Large magnetic anomalies are found over the plateau, many of which are generated by basement topography. Remanent reversal stripes cannot be identified with certainty. Magnetic models that incorporate the basement relief suggest that the basement material is basalt. The magnetic results support the refraction interpretation of a volcanic constitution. The Agulhas Plateau was apparently formed during or after the separation of the Falkland Plateau from southern Africa.