We examined the occurrence of blowfly larvae Trypocalliphora brauneri in Swedish willow warbler Phylloscopus trochilus nestlings at three nearby breeding sites in Central Sweden. The occurrence of parasites was examined in nestlings by visual inspection during ringing. Larvae were encapsulated in the wings, head, neck, or the sides of the body of the nestlings and caused large swellings. Some of the larvae had already left the site of burrowing by an opening in the skin that was visible as a dark crater surrounded by dry skin. Tissue near the encapsulated larvae was swollen and showed clear symptoms of inflammation, in some cases causing deformation of wings. We observed parasites at two of the three sites in 5 out of a total of 36 nests ( 14%) in two years, corresponding to 20 (9.6%) out of 208 nestlings inspected. In nests with parasites 74.1% of the chicks were infested, each containing one to five parasites. No nestlings were found dead in the nests at the date of inspection.