The Santa Maria Basin in southern California is a lowland bounded on the south by the Santa Ynez River fault and on the northeast by the Little Pine-Foxen Canyon-Santa Maria River faults. It contains Neogene sedimentary rocks which rest unconformably on a basement of Cretaceous and older clastic rocks. Analysis of over 4 000 gravity stations obtained from the Defense Mapping Agency suggests that the Bouguer anomaly contains a short-wavelength component arising from a variable-density contrast between the basin's Neogene units and the Cretaceous basement. A three-dimensional inversion of the short-wavelength component (constrained by wells drilled to basement) yields a structure model of the basement and the average density of the overlying sediments, assuming that the basement does not contain large-scale density variations. The density anomalies modeled in the Neogene sediments, showing higher densities in the basin troughs, can be related to diagenetic changes in the silica facies of the Monterey and Sisquoc formations. The basement structure model shows the basin as composed of parallel ridges and troughs, trending west-northwest and bounded by steep slopes interpreted as fault scarps. The basin is bounded on the west by a north-south trending slope which may also represent a fault scarp.