A seismic refraction experiment along a m across the Lesser Antilles island arc in the eastern Caribbean has shown that the crust beneath the arc contains two refractors with average seismic velocities of 6.2 and 6.9 km/s. The upper, 6.2 km/s refractor varies in depth and shows large fluctuations in its seismic velocity. The layer betweeen the refractors has an average thickness of about 10 km, and varies between 2 and 20 km. The total thickness of the crust of the arc is about 30 km beneath Martinique and about 35 km beneath St Vincent. The approximate depths of the Moho beneath the centres of Grenada and Tobago Troughs either side of the arc are 21 and 25 km respectively. Both these troughs, although of different origin, are underlain by crust of modified oceanic type.