This report provides a detailed ultrastructural study of the life cycle, including proliferative and sporogonic developmental stages, of the first Pleistophora species (microsporidium) obtained from an immune-incompetent patient. In 1985, the organism obtained from a muscle biopsy was initially identified as belonging to the genus Pleistophora, based on spore morphology and its location in a sporophorous vesicle. Since that initial report, at least two new microsporidial genera, Trachipleistophora and Brachiola, have been reported to infect the muscle tissue of immunologically compromised patients. Because Trachipleistophora development is similar to Pleistophora, and as Pleistophora was only known to occur in cold-blooded hosts, the question of the proper classification of this microsporidium arose. The information acquired in this study makes it possible to compare Pleistophora sp. (Ledford et al. 1985) to the known human infections and properly determine its correct taxonomic position. Our ultrastructural data have revealed the formation of multinucleate sporogonial plasmodia, a developmental characteristic of the genus Pleistophora and not Trachipleistophora. A comparison with other species of the genus supports the establishment of a new species. This parasite is given the name Pleistophora ronneafiei n. sp.