Technical Abstract: Relapse is common in immunocompetent and immunosuppressed humans infected with Isospora belli). Relapse is believe to be associated with the presence of extraintestinal stages. Histological, immunohistological, histochemical, and ultrastructural methods were used to examine extraintestinal tissue cysts of Isospora belli in an AIDS patient. Anti-sera made in rabbits to Isospora suis, Toxoplasma gondii, Hammondia hammondi, Sarcocystis neurona, Neospora caninum, and Caryospora bigenetica were tested against I. belli tissue cysts in the avidin-biotin peroxidase complex immunohistological (ABC) test. Most anti-sera reacted positively in the ABC test at dilutions of 1:100 but not at dilutions of 1:250. Some anti-sera to N. caninum and H. hammondia reacted positively at dilutions of 1 :1,000 in the ABC test. Most reactive anti-see stained the tissue cyst wall and not the enclosed zoite. Eight histochemical tests were examined and most were non-reactive with I. belli zoites or tissue cysts. Transmission electron microscopy revealed that the tissue cyst wall was composed of granular material and it was directly beneath the parasitophorous vacuole membrane. Zoites were in the center of the tissue cysts and surrounded by fibrillar material that appeared to originate from the zoite surface. Tubule-like structures were present in the granular tissue cyst wall and in the fibrillar material that surrounded the zoite. Zoites contained a crystalloid body.