This is a story about hyperparasitism: flies that live as parasites on bats in turn serve as hosts for small ectoparasitic fungi, the Laboulbeniales. Laboulbeniales fungi are microscopic, obscure, and understudied, even neglected by the mycological community. The small community of researchers studying Laboulbeniales primarily focus on taxonomy (description of species) and ecology (effects of the fungi on their hosts). With this project, we propose to go one step further and ask broad questions about diversity, host specificity, traits affecting parasitism by Laboulbeniales, and host switches playing a role in the formation of new species. Bat fly-associated Laboulbeniales from the neotropics are seemingly more diverse compared to those Laboulbeniales from the Old World. During the proposed work we will resolve this bias – we hypothesize that there is undiscovered cryptic diversity present in the species of ectoparasitic fungi from the Old World. This means that there may be multiple species present, although not morphologically distinguishable from each other. We know from previous studies that bat roosting behavior has an influence on parasitism by bat flies. We hypothesize that in turn this behavior also influences parasitism of bat flies by Laboulbeniales. Some bat fly-associated Laboulbeniales penetrate their host for nutrition; we think that these species are strictly host specific, whereas fungi that do not penetrate their host are expected to have a wider host range.