Lateral velocity structure in the lithosphere beneath southern California is investigated using relative array analysis. In this form of analysis, P wave polarization values measured by a subset of receivers (subarray) are compared to values measured by the entire set of receivers (array). The effects of lateral velocity structure beneath the subarray can be distinguished from the effects of average velocity structure beneath the entire array. Lateral velocity gradients can be mapped as the subarray window is moved across the network. The method is insensitive to effects of near-source and lower mantle heterogeneity, does not lose resolution near the periphery of the array, and utilizes a locally derived ray geometry. Lateral velocity variations beneath the California Institute of Technology-United States Geological Survey network detected by relative array analysis can be compared directly to velocity anomalies found by inversion of travel time residuals. Two regions near the edge of the tomographic image developed by Humphreys and Clayton are investigated; both regions are experiencing active tectonics. Relative array analysis suggests that relatively high velocity may be present beneath the westernmost Transverse Ranges. This feature extends to a depth of roughly 80 km and may be related to the high velocity upper mantle anomaly imaged beneath the central and western Transverse Ranges. Relatively low velocity is associated with the crust and uppermost mantle south of the Salton Sea. This feature is probably related to hot, ascending mantle but is located east of the surface expression of the spreading axis in the Imperial Valley. ¿ American Geophysical Union 1994