New multichannel seismic reflection data provide details of the structure, stratigraphy, and geologic history of the deep western Gulf of Mexico unresolved by earlier single-channel work. These data show a thick sedimentary section lying on an irregular acoustic reflector thought to be oceanic crust that was possibly formed in late Paleozoic or early Mesozoic time. Above the irregular acoustic reflector, six seismic units are defined on the basis of reflection characteristics and basinwide continuity. One unit containing the salt (Jurassic?) that feeds the Campeche-Sigsbee Salt Dome province can be traced northward toward the Sigsbee Escarpment, but it pinches out against the base of the Campeche Escarpment. The four units lying above the salt unit reflect an extended period of pelagic sedimentation followed by mid-Tertiary to Pleistocene turbidite sedimentation.