Publisher Summary This chapter studies the genus Coccidiascus. In the determination of asexual reproduction, asexual cells are spherical to ovoid, 5–15 μm in length, have a large vacuole and nucleus, and reproduce by budding. Hyphae, pseudohyphae, and ballistoconidia are lacking. In sexual reproduction it is seen that asexual cells transform into elongated asci, which are banana- or crescent-shaped, and arranged within parasitophorous vacuoles of Drosophila intestinal epithelium cells in the manner of coccidian schizonts. Thick-walled asci observed in host tissue range from 13–18 μm tip to tip. Mature asci contain two relatively thin-walled ascospores, closely intertwined in a helix. Asci from dead flies have been observed to contain up to eight ascospores. Ascospores extruded from asci are spindle-shaped, bilaterally flattened, and without apical cytoplasmic appendages, each with an apical nucleus. In physiology and biochemistry it is seen that the single known species of this genus has not been obtained in axenic culture and fermentation, and assimilation reactions are unknown. This chapter further examines the phylogenetic placement. The type of species mentioned is Coccidiascus legeri. The systematic discussion of the species mentioned is type strain.