Multichannel seismic reflection profiles show that most of the western Colombian Basin is underlain by a large oceanic plateau. The crust of this plateau has a smooth upper surface, an oceanic velocity structure, and a thickness at least twice that of typical oceanic crust. Locally well defined reflections within the plateau crust are related to layered basalt flows and interbedded sediments. A mounded reflection pattern within acoustic basement at the crest of the plateau (Mono Rise) may represent volcanic knolls. Smooth-surfaced basement of the western Colombian Basin is probably equivalent to the Upper Cretaceous basement complex below seismic reflection horizon B″ in the central Venezuelan Basin and resembles the basements of some Cretaceous oceanic plateaus and aseismic rises in the western Pacific. An area of apparently typical oceanic crust lies adjacent to the Colombian Basin plateau and may represent relatively unmodified oceanic crust upon which the plateau was constructed. Reflections within the flank of the Colombian Basin plateau appear to overlap this crust, indicating that the plateau is younger than the oceanic crust. Oceanic plateau crust extends to Panama and Costa Rica without apparent structural break and may be continuous with the basement complex of southern Central America.