A three-dimensional P wave velocity model of south-central California from the Coast Ranges to the Sierra Nevada shows that the crust under most of the southern Sierra Nevada batholith has seismic velocities (5.9--6.3 km/s) below the continental average. The crust is not much thicker (on average about 35 km) than in the adjacent Great Valley and Basin and Range province apart from a small, northward thickening, crustal root under the western Sierra Nevada that reaches a depth of 42 km. Crustal velocities above the continental average are observed beneath much of the Great Valley due to a high-velocity body underlying the sedimentary basin and the Foothills metamorphic belt (6.4--7.0 km/s). Upper mantle velocities are generally low (7.8 km/s) but span a wide range (7.4--8.2 km/s). We display the velocity model in several cross sections and maps of Moho depth and average crustal velocity. The measured velocities in the upper and mid crust of the Sierra Nevada batholith are in good agreement with laboratory measurements on Sierra Nevada tonalites after corrections for density and temperature. Peridotite xenoliths from the eastern Sierra Nevada suggest strong upper mantle anisotropy, which could explain some of the velocity heterogeneity in the Sierra Nevada mantle. By the time Cretaceous subduction-related magmatism ceased, the Sierra Nevada arc must have had a thick mafic lower crust; yet a principal result of our work is that today the batholith has a crust of mainly felsic composition throughout. A subcrustal layer with velocities below normal Pn velocities (<7.6 km/s) may indicate the presence of lower crustal material in eclogite facies. ¿ 2000 American Geophysical Union