Abstract
Enterocytozoon bieneusi is the most common microsporidian parasite found in humans with acquired immunodeficiency syndrome. A nearly identical organism was recently recognized in rhesus macaques (Macaca mulatta). Ultrastructural examination of this microsporidian parasite in biliary epithelium of rhesus macaques reveals characteristics unique to E. bieneusi, including 1) a lack of sporophorus vesicles or pansporoblastic membranes, 2) direct contact of all stages with the host-cell cytoplasm, 3) elongated nuclei present within proliferative and sporogonial stages, 4) late thickening of the sporogonial plasmodium plasmalemma, 5) electron-lucent inclusions present throughout the life cycle, 6) precocious development of electron dense discs before plasmodial division to sporoblasts, and 7) the presence of polar tube doublets within spores and sporoblasts visualized as 5–7 coils in section.