We studied age-dependent patterns of flea infestation in 7 species of rodents from Slovakia (Apodemus agrarius, A. flavicollis, A. sylvaticus, A. uralensis, Clethrionomys glareolus, Microtus arvalis, and M. subterraneus). We estimated the age of the host from its body mass and expected the host age-dependent pattern of flea abundance, the level of aggregation, and prevalence to be in agreement with theoretical predictions. We expected that the mean abundance and the level of aggregation of fleas would be lowest in hosts of smallest and largest size classes and highest in hosts of medium size classes, whereas pattern of variation of prevalence with host age would be either convex or asymptotic. In general, mean abundance and species richness of fleas increased with an increase in host age, although the pressure of flea parasitism in terms of number of fleas per unit host body surface decreased with host age. We found 2 clear patterns of the change in flea aggregation and prevalence with host age. The first pattern demonstrated a peak of flea aggregation and a trough of flea prevalence in animals of middle age classes (Apodemus species and C. glareolus). The second pattern was an increase of both flea aggregation and flea prevalence with host age (both Microtus species). Consequently, we did not find unequivocal evidence for the main role of either parasite-induced host mortality or acquired resistance in host age-dependent pattern of flea parasitism. Our results suggest that this pattern can be generated by various processes and is strongly affected by natural history parameters of a host species such as dispersal pattern, spatial distribution, and structure of shelters.