We studied the prevalence of a louse fly (Olfersia fumipennis) in a dense breeding colony of Ospreys (Pandion haliaetus) at San Ignacio Lagoon, Baja California Sur, Mexico. We determined fly prevalence and infestation intensity of 45 nestlings (age 32–59 d). We found louse flies in 65% of the nests sampled (n = 34). Louse fly prevalence in nestlings (n = 45) was 56%, the highest rate reported for any raptor species. Male and female nestlings showed similar levels of fly prevalence and infestation, possibly because the numbers of each sex in this population were similar. We found that louse flies had a spatially dispersed distribution, such that the proximity of any nest to any other within the colony did not influence the prevalence of louse flies nor the louse fly load. The productivity of nests with parasitized young (1.3 ± 0.5 young/nest) was similar to that of nests with young that were not parasitized (1.4 ± 0.6 young/nest; P > 0.05). In terms of spatial location, the overall productivity (number of young per successful nest) of the colony was affected by louse fly prevalence. The condition of coloniality (i.e., high density of nests) likely caused high overall louse fly prevalence. A high prevalence of parasitism might be part of the cost of nesting in colonial conditions. Additional studies on the prevalence of louse flies and the health and body condition of nestlings are needed to evaluate the health of Osprey populations.