Patterns of aggregation of species or individuals may result from combinations of interspecific interactions such as competition, facilitation, or apparent facilitation, as well as from equivalent responses to environmental factors. Host–parasite systems are ideal for the investigation of mechanisms that structure assemblages. Interspecific aggregation is documented for multiple groups that are ectoparasitic on mammals and host-mediated apparent facilitation has been suggested to explain these aggregation patterns. To investigate the generality of this pattern and to determine likely structuring mechanisms, I analyzed species co-occurrence, correlations of abundances, and nestedness for ectoparasite assemblages from each of 11 species of Neotropical bat. Ectoparasite assemblages on four of 11 host species exhibited significant positive co-occurrence for the entire assemblage or for at least one pair of species in the assemblage; ectoparasites on two host species exhibited positive co-occurrence that approached significance. There was no evidence of negative co-occurrence. Nine species-pairs exhibited positive abundance correlations, including seven of the eight species-pairs that exhibited positive co-occurrence. No species-pair exhibited a negative correlation of abundances (i.e. density compensation). Ectoparasite assemblages from five of 11 host species exhibited nestedness, including all three assemblages that exhibited assemblage-wide positive co-occurrence. Multiple mechanisms associated with host characteristics may contribute to host aggregation in ectoparasite assemblages, including host body size, vagility, home range size, burrow or roost size and complexity, immunocompetence and social structure. In general, data in this study and elsewhere are not consistent with interspecific interactions among ectoparasites, including apparent facilitation, being primary structuring mechanisms of ectoparasite assemblages on mammalian hosts. Rather, host behavior and ecology are likely to affect the frequency of host–ectoparasite encounters and of conspecific host interactions that facilitate transfer of ectoparasites, thereby, molding patterns of ectoparasite co-occurrence, abundance and species composition on mammalian hosts. Combinations of characteristics that are primarily responsible for molding ectoparasite assemblage composition likely are host-taxon specific.