The dispersal of the nest parasite Crataerina pallida Latreille 1812 (Diptera: Hippoboscidae), between nests of their common swift, Apus apus (Aves: Apodidae), hosts is reported for the first time. Adult parasites were marked, and their presence in other nests determined. More than 10% of marked parasites were found in nests other than those in which they were originally marked, indicating that such dispersal is frequent. Because the supposed lack of dispersal of this parasite between common swift hosts that are unrelated has been used to account for its apparent avirulence upon hosts, this discovery has important implications for understanding of this host—parasite system and parasitic virulence. New explanations accounting for its apparent lack of virulence may be required. The results of this preliminary study indicate avenues for more in-depth research of this and other host—parasite systems.