A new genus and species of parasitic bat fly, Megistapophysis mordax gen. n., sp. n., is diagnosed and described within the subfamily Trichobiinae (Diptera: Hippoboscoidea: Streblidae). Illustrations of the wing, habitus, and male genitalia are provided. The new genus has affinities to species of Megistopoda Macquart, 1852, Neotrichobius Wenzel & Aitken, 1966, Paratrichobius Costa Lima, 1921, and the Trichobius phyllostomae group of species. Morphological affinities of related taxa to the new genus are discussed, and information regarding ecology and host associations is presented. The new genus is known exclusively from Costa Rican Sturnira mordax (Goodwin, 1938) (Chiroptera: Phyllostomidae: Stenodermatinae).