Populations of successfully established exotic organisms rarely reach noticeable densities in their new environment, and little information is currently available on the population dynamics of the exotic insects released on the Hawaiian islands. In the present study, we surveyed present incidence of a tephritid fly of New World origin, Tetreuaresta obscuriventris (Loew.), on the island of Kauai. This fly was introduced to the island in 1961 for the biological control of an important weed species, Elephantopus mollis Kunth. Between 80 and 90% of flowerheads collected from E. mollis Kunth in our survey contained immature T. obscuriventris. An average infested flowerhead contained approximately 5.2 fly larvae. Fly populations in the surveyed areas followed aggregated distribution, and the mean number of flies per infested flowerhead was positively correlated with the percent of infested flowerheads.