A potential citrus pest, the Old World swallowtail butterfly Papilio demoleus L. was recently discovered in the Dominican Republic; however, there are many subspecies of this butterfly and not all are considered pests of citrus. We used genetic markers to determine the source population (subspecies), by comparing sequences from butterflies collected in the Dominican Republic with individuals whose provenance was known, and to indicate whether single or multiple introductions had taken place. Sequences of up to 1,400 nucleotides from the mitochondrial cytochrome oxidase subunit I gene were characterized from 28 specimens collected in the Dominican Republic and compared with known P. demoleus sequences available from GenBank. We confirmed earlier estimates, based on morphology, that the butterflies had originated in Southeast Asia, where the species is known to be a citrus pest. Lack of sequence variation among specimens from the Dominican Republic suggests that a single introduction in early 2004 was most likely. It is not known bow the butterflies reached the Dominican Republic.