In this study, the physiological and behavioural factors that influence infestation levels of wing mites on a large sample of Daubenton's bat (Myotis daubentonii) were investigated. This is the first study to present a comprehensive data set that includes male M. daubentonii over the entire seasonal activity period. In males, parasite load increased during spermatogenesis, which can be explained by high testosterone levels acting as an immunosuppressant. Prevalence rates and infestation levels in males were always lower than those of females and juveniles, possibly due to grooming behaviour which may be energetically restricted in females and inefficient in juveniles. Colony size did not appear to influence parasite load in males, but was found to have an influence in females and juveniles. Larger group sizes in males might not increase parasite load since grooming behaviour and roost switching are frequent. This study showed that parasite load in free-ranging Daubenton's bats is sex-, season- and age-specific due to variations in behaviour and physiology.