The hemlock woolly adelgid, Adelges tsugae (Homoptera: Adelgidae), is an introduced pest of unknown origin that is causing severe mortality to hemlocks (Tsuga spp. ) in eastern North America. Adelgids can also be found on other Tsuga species in western North America and east Asia but these trees are not significantly damaged. The purpose of this study is to use molecular methods to clarify the relationship among hemlock adelgids worldwide, and therefore determine the geographic origin of the introduction to eastern North America. A 541 base pair segment of the mitochondrial COII gene was sequenced for adelgid samples collected from hemlock in multiple locations in eastern and western North America, China and Japan. Phylogenetic analyses indicate that the source of A. tsugae in eastern North America was a population of adelgids living on T. seiboldii in Japan. Adelgids collected in China appear to represent a separate lineage, as do adelgids collected from the other Japanese hemlock species, T. diversifolia, and from T. heterophylla in western North America. Implications for hemlock woolly adelgid control, adelgid taxonomy, and plant-insect co-evolution will be discussed.