Three gravity profiles were completed across the inferred trace of the Santa Ynez River fault in southern California. This fault is believed to be the structural boundary between the Santa Maria Basin and the western Transverse Ranges to the south. A large, steep gravity gradient is associated with the inferred east-west trace of this fault. Gravity to the north of the fault is approximately 60 mGal lower. The gravity anomalies were analyzed to answer two questions: What density contrasts (and therefore rock types) are juxtaposed across the fault at depth? What is the geometry of the fault at depth? Forward and inverse modeling was performed on the anomalies using structural constraints from geologic cross sections and a seismic refraction study plus density constraints from well logs. The Bouguer gravity was modeled as a combination of three sources: a deep long-wavelength contribution from the slight northward deepening of the Moho; an upper crust contribution from the density contrast between the low-density Miocene Monterey and Sisquoc formations (present in the Santa Maria Basin) and the denser shallow basement beneath the western Transverse Ranges to the south; and a high-density midcrust contribution, extending from 3 to 11 km depth below the Transverse Ranges. The origin of the midcrust contribution is unknown, but we suggest that it may arise from a layer of oceanic crust beneath the western Transverse Ranges south of the Santa Ynez River fault. ¿ American Geophysical Union 1989