Feather mites are ubiquitous avian ectoparasites whose population structure is expected to vary with degree of coloniality and other aspects of host bird species' ecology. We investigated aggregation . and infrapopulation structure of feather mites of the genus Alloptes Canestrini on four auk species (with different levels of nesting aggregations) breeding at the Gannet Islands, Labrador, Canada. Feather mite populations were more prevalent and had higher relative abundance in Razorbills, Alca torda Linnaeus, and Atlantic Puffins, Fratercula arctica Linnaeus, compared to murres (Uria spp.). Prevalence increased moving from highly aggregated ledge-nesting hosts (53% in murres) to less aggregated, burrow-nesting hosts (87% in puffins), with similar trends in relative abundance. Prevalence and relative abundance of individual age-classes of feather mites varied among host species without any clear relationship to host coloniality. Infrapopulations were strongly female-biased (4–36% males), with a largeproportion of tritonymphs and adult females. Precopulatory mate guarding was observed in all sevencases of coupled individuals, each involving an adult male and a female tritonymph. This study constitutes the first record of precopulatory mate guarding in the genus Alloptes, a male strategy to ensure insemination of adult females as soon as they molt. Female-biased sex ratios may be produced in order to reduce local mate competition among male feather mites. Our study revealed several nonintuitive patterns of mite population structure, underlining the need for more detailed work on these ecologically important but poorly known species.