By presenting race-specific population concentration indexes for U.S. counties for the 1950–1980 period, this paper updates and extends previous analyses of 19001974 patterns of population concentration by Duncan et al. (1961) and Vining and Strauss (1977). Declines in population concentration persisted through 1980 only among the white population. Blacks continued to experience increases in population concentration between 1970 and 1980. These racially divergent patterns of concentration in the 1970s have translated into increases in segregation between blacks and whites across U.S. counties.