The microsporidium Cystosporogenes deliaradicae sp. no v., a parasite of the adipose tissue of adult cabbage root flies, Delia radicum in Denmark, is described based on light microscopic and ultrastructural characteristics. All life cycle stages have isolated nuclei. The sporogony is polysporoblastic, yielding 4–16 (most commonly 6 or 8) ovocylindrical spores, measuring 1.2–1.5 × 4.6–5.5 μm when fixed and stained. The spore wall has an approximately 43 nm thick, layered exospore with a median layer resembling a double membrane. The polaroplast is uniformly lamellar. The isofilar, 114–123 nm thick, polar filament is arranged in 11–15 (most commonly 12–13) coils in a single layer close to the spore wall. The angle of tilt is approximately 35°. The last coil touches the posterior pole of the spore. A sporophorous vesicle is produced by the sporont. The envelope is generated as a duplication of the plasma membrane. Typically the vesicle primordia grow from one pole of the sporont to the other. The episporontal space initially has granular inclusions. Later septate, persistent tubules of exospore material appear. Simultaneously with the release of sporoblasts, wide tubulus-like, non-persistent structures are formed. They are covered by regularly spaced electron-dense material. Anomalous sporogony was frequently observed, probably caused by a simultaneous parasitism by the fungus Strongwellsea castrans. The systematic position of the microsporidium is discussed, including the reasons for incorporating the genus into the family Glugeidae.