An unidentified coccidium was found in the brain and the heart of a 6-year-old female cat. Tissue cysts measured 40-130 x 40-112 microns and contained hundreds of bradyzoites measuring 4.2-5.2 x 1.2-1.8 microns. The cyst wall was 0.26-0.53 microns thick. Tissue cysts reacted positively with anti-Toxoplasma gondii serum but not with anti-Neospora caninum serum in an immunohistochemical test. Ultrastructurally, the cyst was aseptate and contained banana-shaped bradyzoites. Micronemes and rhoptries in bradyzoites were unlike those found in T. gondii bradyzoites. In the unidentified coccidium, the micronemes were arranged in rows and the rhoptries extended from the anterior tip to the posterior end.