This paper addresses the ecological problem of how a territorial system is organized around a metropolitan center or system of such centers. We hypothesize that accessibility to the metropolitan system is inversely related to the degree of internal heterogeneity or differentiation of nonmetropolitan cities. Measuring differentiation on four variables: industry, occupation, education, and income, we find general support for this proposition. Ecological theory bearing on these problems also suggests that such specialization derives from trade interdependencies that grow with increased accessibility to the total territorial system. We interpret this to mean that the degree of industrial differentiation in a community mediates the relation between accessibility and other dimensions of differentiation. Again, the evidence seems to support this prediction.