Abstract. Biological aggregations such as insect swarms and bird flocks may arise from a combination of social interactions and environmental cues. We focus on nonlocal continuum equations, which are often used to model aggregations, and yet which pose significant analytical and computational challenges. 1. Introduction. This paper has three primary aims. First, beginning with a partial integrodifferential equation model of biological aggregation, we show that a long-wave approximation yields a degenerate Cahn-Hilliard equation akin to models describing phase separation in materials science and evolution of thin fluid films. Second, we study solutions of the degenerate Cahn-Hillard model and demonstrate that they compare well with those of the original model. Using the Cahn-Hilliard model is advantageous because it eliminates many of the analytical and computational challenges of nonlocal equations, particularly in higher dimensions. Our methodology is to study the energy minimizers of this local equation, rather than studying the time evolution of the original nonlocal equation. Finally, we use the Cahn-Hilliard model as a testbed to assess whether imposing an external potential modeling the environment, e.g., food sources, can be used to control peak density in aggregations, which is of interest for locust swarms.Biological aggregations such as bird flocks, fish schools, and insect swarms are driven by social interactions between group members. Typical social forces include attraction, repulsion,