During two seasons insects that visited flowers of Aristolochia pilosoa were studied in Las Cumbres, Republic of Panama. The vast majority of visitors were Diptera, notably Milichiidae (942 individuals, 18 species in 7 genera) and Chloropidae (97 individuals, 7 species in 5 genera). Occasional visitors, probably nonpollinators, included several other kinds of Diptera, a few wasps, an ant, a few Homoptera, and a spider. No significant differences in relative abundance of the visiting insect species were observed between two very different forms of A. pilosoa growing side by side. The fimbriae on the flag of the flowers are important for long-distance attraction of flies, but the conspicuous purple markings seem to be irrelevant. At shorter range attraction to the floral tube overrides that of the flag, and the flies struggle their way down the tube among a dense mat of trichomes. All Milichiidae and Chloropidae captured were females. It is not clear what, if any, benefit accrues to the flies from being incarcerated repeatedly in Aristolochia flowers, but if they obtain some nutrient or other substance, it may be needed for the development of the ovaries. MEMBERS OF THE FAMILY ARISTOLOCHIACEAE share a peculiar mode of pollination. Insects, mostly Diptera, many of them covered with pollen from previous entrapment, enter the tube of the flower, which often has trichomes (permitting only one-way traffic), and become trapped in the inflated flowerbase (utricle), where they pollinate the then pistillate flowers. The insects remain trapped until the stamens ripen and cover the insects with pollen and the trichomes in the floral tube have wilted (Muller 1883, Pfeiffer 1960, Daumann 1971, pers. obs.). Insects are attracted to Aristolochia flowers by their odor (Daumann 1971, Brantjes 1980). Visitors reported for Aristolochia clematitis consist mostly of Diptera of the families Chironomidae, Ceratopogonidae, and Bibionidae (Muller 1883; Daumann 1971; Havelka 1978, 1983). Brantjes (1980) reports Lauxanidae and Phoridae as pollinators of Aristolochia melanostoma and Chloropidae and Milichiidae as pollinators of Aristolochia arcuata, both in Brazil. The present paper reports visitors to flowers of A. pilosa