The purpose of this study is to describe and interpret the ecology of mortality in a metropolitan area in terms of the structure of population aggregates. Explanations for both the statistical distribution of mortality rates and the processes by which individuals become mortality cases are considered. The data are analyzed according to socioeconomic status, Shevky-Bell social rank and family status, geographical mobility, racial status, and residential conditions. Several hypotheses are suggested in interpretation of the research findings, based on (1) social change, (2) population drift, (3) genetic inheritance, (4) interaction problems, (5) cultural differences, (6) differentials in medical assistance, (7) adoption of the sick role, and (8) societal reaction. Finally, it is suggested that the pursual of a dual research strategy of rates and individual behavior will ensure a more integrated and comprehensive sociological theory of mortality.