Bouguer gravity anomalies in the Transverse Ranges of southern Calfiornia show a pronounced asymmetry in north-south profiles across the mountains and a characteristic shape which is largely insensitive to variations in the topographic load along strike. While individual gravity/topography profile pairs could be consistent with a number of isostatic compensation mechanisms, the ususual relationship between gravity and topography in the San Bernardino Mountains severely limits the class of acceptable models if the Bouguer gravity field is caused by flexure of the crust/mantle boundary. The simplest solution consistent with the data in the Western Transverse Ranges, the San Gabriel Mountains, and the San Bernardino Mountains models the Great Valley and Mojave provinces to the north as a 50-km-thick elastic plate supporting the mountains which are underthrust by a much weaker plate to the south. The best fit to the gravity data in the San Bernardino Mountains requires a terminal bending moment on the southern plate beneath the mountains. The magnitude of the thermal bending moment decreases to the west in the San Gabriel and Western Transverse Ranges. We equate the load responsible for this terminal moment with the high-velocity, slablike feature mapped in the upper mantle beneath the Transverse Rangers using seismic tomography. The coupling between the subsurface load and the overlying lithosphere suggests that the slab represents lithospheric subduction with an extremely small denisty contrast rather than a small-scale convective feature.