A new genus, Vittaforma n.g. is proposed for the human microsporidium Nosema corneum Shadduck, Meccoli, Davis & Font, 1990, based on the ultrastructure of developmental stages in the liver of experimentally infected athymic mice. The diplokaryotic arrangement of the nuclei is the only character that conforms with the description of the genus Nosema. Sporogony is polysporoblastic, sporonts are ribbon-shaped, constricting to give rise to linear arrays of sporoblasts and each parasite is enveloped by a complete cisterna of host endoplasmic reticulum. Comparison of N. corneum, with established genera revealed that there were none with the same combination of characters. Consequently it is proposed that Nosema corneum be placed in a new genus as Vittaforma corneae n. comb.