Metropolitan communities in the United States are interdependent. Each metropolis serves the economy through its key functions while at the same time it depends on other metropolises and systems for some of its own needs. Using 1960 data for SMSAs, this paper demonstrates that a community's position in this differentiated system influences the characteristics of its population. Two perspectives for determining a metropolitan community's key functions are used: one considers economic activities one at a time; the other considers the entire configuration of these activities at once. These two procedures are compared in their ability to identify groups of communities having distinctive social structures, as indicated by occupational and educational composition, income level, age and sex structure, and ability to attract and hold migrants.