In the Pacific region, the African big-headed ant, Pheidole megacephala, is now widespread in tropical areas; populations are also found at higher latitudes in Australia, New Zealand, and Japan. On most inhabited tropical islands in the Pacific, P. megacephala is well known as a household and agricultural pest. Because P. megacephala does not attack humans, this species is often not recognized as an important threat. The negative ecological impact of P. megacephala, however, may be greater than that of any other invasive ant species. In areas where it occurs at high density, few native invertebrates persist. Loss of invertebrate species that serve key functions in the natural community (e.g., important prey species) may have cascading effects leading to the subsequent loss of additional species. Pheidole megacephala tends to thrive in open, disturbed habitats with weedy vegetation that can support high densities of plant-feeding Hemiptera, which these ants tend for honeydew. Before 1900, P. megacephala was known in the Pacific region only from Aru Island (Indonesia) and Hawai‘i. By the 1930s, it was found through much of Pacific Asia, Melanesia, and Polynesia, but it was not collected in Micronesia until 1950. Currently P. megacephala is known from virtually every tropical island group in the Pacific but not from many islands within the groups, particularly uninhabited islands. Quarantine efforts might be successful in keeping P. megacephala off these islands. Because P. megacephala does not commonly dominate areas with intact natural vegetation, setting aside relatively undisturbed habitat on inhabited islands may also be effective in protecting native invertebrates from attack by this ant.