Assembly rules predict that species will co-occur less often than expected by chance and those that do co-occur will differ morphologically. Streblid bat flies and their hosts provide an ideal system with which to test predictions of competition and coexistence ecology because many bat species host multiple fly species. Thirty-one Venezuelan bat species were parasitized by two or more bat fly species, and individuals of different fly species often co-occur on host bat individuals more often than expected by chance. Three ecological groups of flies are defined based on site specificity and evasion behavior. Flies were then characterized morphometrically and data were subjected to multivariate analyses. The ecological groups were found to differ significantly in terms of their overall morphology. Three ecomorphological groups of bat flies are defined. Co-occurring fly species almost always comprise different ecomorphological groups. Ecosynoxenous associations are defined and discussed, and it is argued that morphological hyperdispersion in part facilitates multiple species co-existence of bat flies on their hosts.