The avifauna of the Galapagos islands is characterized by a small number of endemic species, including the 13 species of Darwin's finches. The introduced fly parasite Philornis downsi reduces nestling survival and growth rate of altricial birds, and can cause mortality and morbidity of the nestlings. We examined the occurrence of Philornis downsi among islands and at different elevations. The parasite was found in nests from 11 of 13 islands sampled. The two islands on which P. downsi was not found were Española and Genovesa, both arid islands with no humid highlands and distant from the centre of the archipelago. Parasite infection intensity was greater in nests at higher elevations, and on islands that have moist highlands, which may serve as a reservoir for the files. A full understanding of the fly's ecology may permit the development of eradication or control methods, or at least mitigation of its effects on the birds.