Having studied 67 birds of six species of the family Corvidae, Sarcocystis cysts were found in 16 (23.9%) individuals belonging to three species. The highest prevalence of infection (35.9%) was determined in the hooded crow (Corvus cornix). Two types of sarcocysts, which were temporarily called cysts type I and type V, were determined in the corvids examined. By light microscope, type I cyst wall seemed to be thin (< 1.0 microm) and smooth. Banana shaped cystozoites measured 6.0-8.0 microm in length. By light microscope, type V cyst wall seemed striated and reached up to 2.5 microm. Banana shaped cystozoites measured 6.1-7.9 x 1.4-1.8 microm. Ultrastructurally, the cyst wall amounted to 2.1 microm and had stump-like protrusions that differed greatly in size and shape. The parasitophorous vacuolar membrane had indentations and clearly visible (up to 0.2 microm in length) microprojections, which also differed considerably in size and shape. The ultrastructure of type V cyst wall differed from all those Sarcocystis spp. described thus far. On the basis of this-Sarcocystis cornixi sp. nov.-is proposed for this type of sarcocysts. Partial sequences of 18S rRNA and 28S rRNA genes were determined for this species and a phylogenetic analysis of the Sarcocystidae family was performed. In the phylogenetic tree, S. cornixi is grouped together with Frenkelia microti, F. glareoli, S. muris, S. neurona and the unnamed Sarcocystis species whose intermediate hosts are birds. S. cornixi is the most closely related to Sarcocystis sp. (cyst type I) from the white-fronted geese.