Seismic velocities and layer depths were computed from 26 air gun/sonobuoy profiles made along a southwesterly traverse of the Colombia basin. Penetration to the deeper crustal layers and the upper mantle was achieved with a large air gun sound source. The crustal structure of the Colombia basin is a complex arrangement of basement ridges and interposed basins that is masked by an overburden of thick, flat-lying sediments. Earlier two-ship refraction stations were inadvertently located on the basement ridges, which had led to the mistaken conclusion that the basin contains unusually thick crust. Mantle refractions from two sonobuoy profiles in the undisturbed parts of the basin suggest a normal oceanic crust that has been depressed by the addition of about 3 km of sediment. The reflection records from the Colombia basin show that the basement rocks have considerable relief, unlike the smooth B\ (acoustic basement) surface in the Venezuela basin. Furthermore, there is no consistent reflector in the sediments that can be correlated with horizon A\ in the Venezuela basin. Sound velocity in the sediments increases at a normal rate south of about 14¿N, but to the north where there is no intermediate (horizon A\ type reflector, the rate of increase is unusually high, owing to a thinner overburden on a high-speed sediment layer. Accordingly, the horizon A\ and B\ usage is not followed in the present work in the Colombia basin.