Microscopic examination of faecal samples collected from naturally infected blackcap warbler, Sylvia atricapilla revealed the presence of a new Isospora species; I. ashmoonensis. Oocysts were subspherical to oval, measuring 24.0 (21.0-25.0) microns in length and 20.0 (19.0-20.5) microns in width. A micropyle and an oocyst residuum were absent whereas one polar granule was seen. Ellipsoidal sporocysts contained a stieda body and measured 17.0 (16.0-18.5) x 9.2 (9.0-10.0) microns. Sporulation took place within 50 hrs. at room temperature. Developmental stages of the parasite in epithelial cells of the posterior third of the ileum were also described.