Geophysical investigations of the northern Somali Basin and the Seychelles-Mauritius Ridge conducted aboard R.V. Chain of the Woods Hole Oceanographic Institution are described and some results presented. Gravitational and total magnetic fields and bathymetry were measured continuously, and continuous seismic reflexion profiles were recorded over a major portion of the track. Cores, dredge samples, heat flow measurements, and underwater photographs were also obtained. It is considered that the northern portion of the Somali Basin is a deep sedimentary basin partially enclosed to the east by a submarine ridge from which alkaline gabbro has been dredged and to the south by partially buried abyssal hills. On the evidence from seven crossings of the Seychelles-Mauritius Ridge, it is proposed that the Ridge comprises two sections. The northern section, composed of nearly horizontally stratified rocks, extends from near the northern part of Saya de Malha Bank to the Seychelles Platform. The southern section is a linear, probably volcanic ridge that extends from north of Mauritius through Saya de Malha Bank, and may continue as a subsurface feature to the northeast. The two sections abut near Saya de Malha Bank, forming a continuous topographic feature.