The adventive cactus moth Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae), a widely used biological control agent for Opuntia Mill. cacti, was detected in Florida in 1989. Since then, it has spread along the Atlantic and Gulf Coasts of southeastern United States, threatening native Opuntia populations. We examined the phylogeography of 20 C. cactorum populations from Australia, South Africa, Hawaii, the Caribbean, Mexico, and the southeastern United States based on 769 bp of cytochrome oxidase subunit 1. Five distinct haplotypes were discovered, three of which occur in the United States. Cactoblastis cactorum in the United States falls into two distinct lineages: a western haplotype along the Gulf Coast and an eastern lineage with two haplotypes along the Atlantic Coast, with one of the eastern haplotypes identified as occurring at a single locality on the Gulf Coast. The two lineages have nontrivial genetic divergence (0.5%), and both are more closely related to populations outside the United States than they are to each other. This leads us to conclude that C. cactorum has been introduced to the United States at least twice. The isolated eastern haplotype on the Gulf Coast may indicate that C. cactorum has been introduced a third time, either from the Atlantic Coast or from outside the United States. Evidence from analysis of haplotypes and other information indicates that dispersal by commercial import action and human transport may be more important than flight ranges of ovipositing females for determining long range expansion of the species. Interestingly, the east-west pattern mirrors other coastal species distributions that have been interpreted as being due to Pleistocene vicariance.