Animal migrations offer a unique opportunity for developing and testing hypotheses about the ecological requirements of different species and the tradeoffs that they make between conflicting life-history demands. There has been relatively little research into the causes and consequences of migrations by fruit bats, despite their potential significance for pollination and seed dispersal. We assessed the causes of one of the most spectacular migrations of fruit bats known: the annual influx of an estimated 5–10 million E. helvum into Kasanka National Park in Zambia. We tested several predictions based on the hypothesis that E. helvum migrates to exploit seasonal variations in food supply opportunistically. Phenological data, feeding observations and monitoring of fruit bat movements provided the first quantitative evidence in support of the hypothesis that the migration of E. helvum in Zambia is driven by food supply. The E. helvum colony exhibited several surprising behaviors, including a tendency for migratory satellite colonies to aggregate, rather than to disperse, during the time of peak food production, and a tendency to fly well beyond the most immediate food sources when foraging. Alternative hypotheses to explain the E. helvum migration were not supported, but further research is needed to clarify the results of this preliminary study. Both the size of the colony and its potential for large-scale movements suggest that this bat may play an important economic and ecological role over a significant portion of sub-Saharan Africa. Information is still lacking about migration routes, food sources, habitat requirements and the role of migration in disease transmission between colonies of E. helvum.