The purpose of this investigation was to determine seismically the structure of an island-arc-deep-sea trench sequence and its associated interior basin. A series of seismic-refraction profiles were taken in the Eastern Caribbean and were designed to measure the crustal structure of the broad Venezuelan basin of the Caribbean sea, the island arcs of the Lesser Antilles and Greater Antilles, and the Puerto Rico trench. Results showed that the Caribbean basin, although similar in many respects, is significantly different from the standard ocean-basin structure. Material below the major seismic discontinuity has an average velocity of 7.4 km/sec., considerably different from the 8.0 km/sec, generally observed beneath the corresponding discontinuity in the Atlantic basin. Above this discontinuity the crustal velocities are lower than for the corresponding ocean-basin structure, which indicates a somewhat more acidic composition. Both the crustal and overlying sedimentary and igneous materials are thicker in the Caribbean. The seismic structure for the northern part of the Puerto Rico trench is the same as that for the Atlantic, except for a slight thickening of the crust. In general, the structure along the island arc is more complex than elsewhere but shows crustal velocities nearer the surface than in the Caribbean or Atlantic. From these results and their correlation with the known geology of the area and from other geophysical measurements the writers conclude that the entire Caribbean area has been extensively intruded by large bodies of basic magma which have differentiated deep in the mantle and migrated upward. The writers consider that the introduction of this material has been the primary cause of the tectonic effects observed in the area; in particular the island arcs and deep-sea trenches have been formed by horizontal compression along the border of the altered Caribbean and normal Atlantic basin.