We use observations of the Moho-reflected phase PmP to constrain crustal thicknesses and upper mantle velocities in southern California. We stack normalized absolute values of seismograms from local events in time and range bins after aligning on the initial P arrival and applying a range correction to adjust the various source depths to the surface. Although most individual seismograms do not allow accurate determination of a PmP arrival time, imaging the whole data set in this way shows clear PmP arrivals at ranges from about 90 km to over 250 km. PmP--Pg and PmP--Pn differential times can be measured from the image and used to estimate the Moho depth and upper mantle velocity. For southern California, we obtain an average crustal thickness of 28 km and an upper mantle velocity of 7.8 km/s. Next, we map lateral variations by repeating this procedure for stacks of subsets of the data in which the traces are grouped in caps by Moho reflection point. Estimates of Moho depth range from 18 km in the Salton Trough to 33 km beneath the eastern Transverse Ranges and 36 km beneath the southernmost Sierra Nevada. The upper mantle velocities generally increase from southwest to northeast across the region. We also map postcritical PmP/Pg amplitudes which vary by a factor of 4 with the highest amplitudes in the northwest Mojave Desert. Preliminary experiments with stacking SmS arrivals indicate strongly correlated SmS and PmP amplitude variations. These results provide a guide to source-receiver paths that may produce anomalously strong Moho-reflected phases during future earthquakes.¿ 1997 American Geophysical Union