Fungal mycoparasitism—fungi parasitizing other fungi—is a common lifestyle in some basal lineages of the basidiomycetes, particularly within the Tremellales. Relatively nonaggressive mycoparasitic fungi of this group are in general highly host specific, suggesting cospeciation as a plausible speciation mode in these associations. Species delimitation in the Tremellales is often challenging because morphological characters are scant. Host specificity is therefore a great aid to discriminate between species but appropriate species delimitation methods that account for actual diversity are needed to identify both specialist and generalist taxa and avoid inflating or underestimating diversity. We use the Biatoropsis-Usnea system to study factors inducing parasite diversification. We employ morphological, ecological, and molecular data—methods including genealogical concordance phylogenetic species recognition (GCPSR) and the general mixed Yule-coalescent (GMYC) model—to assess the diversity of fungi currently assigned to Biatoropsis usnearum. The degree of cospeciation in this association is assessed with two cophylogeny analysis tools (ParaFit and Jane 4.0). Biatoropsis constitutes a species complex formed by at least seven different independent lineages and host switching is a prominent force driving speciation, particularly in host specialists. Combining ITS and nLSU is recommended as barcode system in tremellalean fungi.