A Encephalitozoon-like microsporidia was found in epithelial cells of the midgut and the salivary glands of Amblyomma cajennense (F.) and Anocentor nitens (Neumann) that had fed on rabbits. Ultrastructural studies demonstrated that all stages of the life cycle of the parasite occur in parasitophorous vacuoles and contain only 1 nucleus. The sporonts detach from the limiting membrane of the vacuole and divide by binary fission to produce the sporoblasts, each presenting a thickened electron-dense wall, and a primordium of a polar filament. Each spore contained a single nucleus, an electron-dense and rough exospore, an electron-lucent and thick endospore, and 5 coils of the polar tubule.