Host-parasite relationships between the Daubenton's bat, Myotis daubentonii Kuhl, 1817 (Chiroptera: Vespertilionidae), and its haematophagous ectoparasite, the mite Spinturnix andegavinus Kolenati, 1857 (Acari: Spintumicidae), were subjected to analyses based on data gathered during a six-year study (1999-2004) within a single study area in South Bohemia, Czech Republic. Seven hundred and fifty-one Daubenton's bats were examined by screening wing membranes with an intensive light source, resulting in 4,690 recorded mites. Sex, age, weight and reproductive state were evaluated for each bat. A body condition index was calculated as a ratio of weight to forearm length. The seasonal course of mite infestation displayed distinct dynamics with the peak during the lactation and post-lactation periods coinciding with occurrence of the most numerous colonies of Daubenton's bats in the study area. Infestation rates differed between the two sexes, being higher in adult females than adult males. Juvenile bats of both sexes (with no differences between males and females) were the most infested group of all. Pregnant females had a significantly higher parasite load than non-pregnant ones, while no differences in infestation rates were found between lactating and non-lactating females. The analyses of the relationship between parasite load and body condition of bats revealed no common trends for all sex- and age-related groups. Two possible explanations are suggested and discussed: (1) There is no true relationship between the two tested variables and, thus, the significant results were attained due to a random statistical effect. (2) Different underlying causal mechanisms may exist that influence parasite load and, especially, body condition, with respect to the particular sex and age category of bats. The seasonal roosting dynamics of the Daubenton's bat are suggested to be the result not only of changing energetic demands of resident population members, but also of coevolutionary strategies within host-parasite relationships.