A new species of fly, Fergusonina giblindavisi Taylor sp. nov. (Diptera: Fergusoninidae), and its associated nematode, Fergusobia ptychocarpae Davies sp. nov. (Tylenchida: Neotylenchidae), are described from flower bud galls on Corymbia ptychocarpa (Myrtaceae). Fergusonina giblindavisi Taylor sp. nov. forms flower bud galls up to about 20 mm in diameter, one of the largest galls recorded for the Fergusoninidae. Unlike other species of Fergusonina Malloch that produce large galls, only one species of Hymenoptera, Megastigmus sp. (Torymidae), a solitary obligate ectoparasitoid, has been reared from F. giblindavisi Taylor sp. nov. The large size and thick epidermis of the flower buds of C. ptychocarpa may preclude many other parasitoids from successful oviposition. A second new species of fly, Fergusonina thomasi Taylor sp. nov., is described from flower bud galls on Corymbia abbreviata. Its associated nematode is unknown.