An ecological conceptualization of the uwrld system of cities is proposed based both on an extension of the ecological rationale for a national system of cities and on insights from uorld-system theory and research. The core concepts are divided into five categories: (1) the key function, (2) hierarchy and dominance-the vertical dimension, (3) specialization-the horizontal dimension, (4) interaction, and (5) dynamics. As a test of selected ideas, the dominance of core international financial metropolises over peripheral South American cities is examined; the measure is the international bank headquarters-branch office link. The hypotheses of ecological dominance were confirmed. The core metropolises dominate the peripheral metropolises of South America, and within the core the upper level metropolises exert greater dominance than the lower level metropolises. The national metropolises of South America are the key intermediaries with international metropolises, but the former have few links among themselves. The world system of cities based on finance seems to be organized independently of national or world regional boundaries. The global expansion of multinational corporations since 1950 has created a network of international economic transactions within and between businesses such as industrial firms, banks, commodity firms, and trading companies. Ecologists argue that these intercommunity transactions are controlled and coordinated by organizations which are concentrated in metropolises; therefore, urban ecological inquiry about the system of cities should include the world scale. The system of cities in the United States, however, has been the focus of ecologists; they have neglected the world system of cities (Wilson). The purpose of this paper is to extend the ecological framework for examining the system of cities to the world scale.