Philornis flies exert strong negative selection on avian host life histories. However, their possible influence at the southern limits of parasite distribution remains poorly studied. We collected data on Philornis parasitism (prevalence, parasite load and latency) during three consecutive breeding seasons of the Masked Gnatcatcher Polioptila dumicola, Vermilion Flycatcher Pyrocephalus rubinus and Blue-and-yellow Tanager Pipraeidea bonariensis in a south temperate forest in Argentina. We hypothesised that Philornis parasitism would negatively influence both the growth and survival of infected nestlings. Regarding body size differences among nestlings of these three host species, we predicted that nestling survival in parasitised nests and parasite load per nestling would be greater the larger the species, and that the number of growth parameters that differ between parasitised and non-parasitised nestlings would be greater the smaller the species. We monitored 564 nests and found prevalence to be 16.7% for the Masked Gnatcatcher, 30.1% for the Vermilion Flycatcher, and 37.9% for the Blue-and-yellow Tanager. Parasitism increased during the season for all three species. As predicted, for the three species, nestling survival was lower in parasitised nests (∼30%) than in non-parasitised nests (∼90%) and for each species, one or more growth parameters were significantly lower for parasitised nestlings compared to non-parasitised nestlings. Our prediction relating to host body size was partially supported since differences in parasite load only emerged in nests that produced fledglings, as well as in the extent of growth parameters affected. This is the first study providing detailed data of Philornis fly parasitism and how this parasite affects the fitness of three bird hosts at the southern limit of the parasite's distribution.—Gonzalez, E., Jauregui, A. & Segura, L.N. (2022). The impacts of parasitic flies (Philornis spp.) on nestlings of three passerines in a southern temperate forest of Argentina. Ardeola, 69: 3-20.