Infestation of marsh tit (Parus palustris, Paridae, Passeriformes) broods by bloodsucking larvae of Protocalliphora falcozi Séguy 1928 (Calliphoridae, Diptera) was studied over an 8-year period in a population breeding in natural holes, in the primeval forest (Białowieża National Park, eastern Poland). Overall 54% of 222 marsh tit nests were parasitized. Prevalence (27–88%) significantly changed across years. This variation did not depend on the timing of the marsh tit breeding season, or winter/spring temperatures. Frequency of infestation did not depend on forest type or hole attributes. Infestation intensity was rather low (median 8, max. 75 flies/nest, 85% of nests with <3 flies/young). Intensity and prevalence were weakly, but positively, correlated. Larger broods contained significantly more blow flies, but per nestling load did not depend on brood size. No effect of infestation on nestlings was recorded – their mortality did not increase, nor was fledging delayed. In response to the presence of blow flies, parents apparently attempted to increase their feeding rate. There was a clear reproductive cost: 60–63% of female and 68–69% of male marsh tits that had none up to eight blow flies, survived to the next spring, but only 34% of females and 44% of males with more than eight flies/brood did so. It is suggested that the small clutch size of marsh tits could have evolved, inter alia, to reduce the fitness costs of ectoparasites.