We invert earthquake P-wave arrival times to image the 3D distribution of P wave velocities in the Mesozoic Peninsular Ranges batholith and nearby areas in southern California. There is a 3% velocity contrast between the eastern and western Peninsular Ranges at 4 and 20 km depth (west side faster) and a 1 to 1.5% velocity contrast across the San Andreas fault zone (south side faster) in the San Bernardino region at 4 to 14 km depth. The San Andreas velocity contrast is due to the juxtaposition of different rock types by slip along the fault zone. The Peninsular Ranges batholith velocity contrast is due to a difference in rock composition across the batholith. The maximum gradient in the crustal velocities is coincident with a compositional boundary within the batholith that reflects emplacement of the batholith across juxtaposed oceanic and continental crust. Quaternary fault development has been in part concentrated at this boundary. ¿ American Geophysical Union 1995.