The main unit of ecological interest is not an individual organism but rather an assemblage of individuals belonging to the same species and coexisting in time and space. Contrary to most free-living species, spatial distribution of parasites is not continuous but consists of a set of more or less uniform inhabited patches represented by the host organisms, while the environment between these patches is decidedly unfavorable and strongly affects the probability of those parasites with free-living stages completing their life cycle and thus persisting. Thus, spatial distribution of an ensemble of conspecific ectoparasites is heterogeneous and fragmented among (a) host individuals, (b) host species within a location, and (c) locations. In this chapter, we will consider the lowest hierarchical level of this fragmentation, namely ectoparasite infrapopulations, i.e., assemblages of conspecific parasites infesting an individual host. We will focus on several common taxa of arthropod ectoparasites of mammalian hosts. We will start with variation in patterns of parasite abundance among parasite species as well as among host species, gender and age cohorts. Then, we will discuss relationships between abundance and distribution of ectoparasites. Finally, we will focus on host-related and environment-related factors affecting ectoparasite abundance and distribution. We will demonstrate that ectoparasite populations are affected by intrinsic and extrinsic factors whose actions promote equilibrium and nonequilibrium conditions, respectively.