Redistribution relative to metropolitan growth of Negro, other non-white and Spanish name populations is examined in Los Angeles County from 1940 to 1960 for a comparable grid of subareas. The subareas are defined relative to their maturity at different time points in order to partially control for population redistribution effects of neighborhood life histories, the spread of older subareas, and the persistence of neighborhood patterns. Shifts in ethnic concentration are shown for both older and newer subareas. Concurrent changes in neighborhood social structures and ethnic populations are described. Findings are categorized under three themes: First, ethnic population increments and redistribution were generally restricted to expanding older subareas. Ethnic populations did not spatially expand at a rate equal to the spread of the metropolis or of older subareas. Second, segregation is greater in both older and newer neighborhoods for Negroes than for other ethnic populations. Negroes experienced the largest proportional increments in both older and newer subareas, as well as the greatest stability in subarea occupancy. Finally, the spatial separation of ethnic populations impedes assimilation in that unique patterns of neighborhood structure come to characterize different ethnic populations, and changes in ethnic composition are reflected in changes in neighborhood social structures.