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Data are presented on the seasonal changes in the population of the louse-fly Crataerina pallida, parasitic on swifts, Apus apus, from the examination of 4094 birds. The swifts arrive to breed in Britain free from this parasite, but the flies soon emerge from pupae in the hosts’nests. The population of adult flies builds up rapidly towards the time when the young birds start to hatch. At this stage up to 70% of the birds trapped were infested. By the time the main hatch of young birds occurs the population on the full grown birds is declining, and continues to decline until flies are absent by the time the birds migrate. Male flies emerge first, and as the season progresses the proportion of males declines and males disappear first. Heavy infestations were frequent with up to 31 flies per bird, but there seemed no relationship between these heavy infestations and poor condition of host (in weight, plumage, etc.). Seven juvenile birds were trapped, but were not heavily infested. Only one fly was found on the 37 first-year birds examined.