A new microsporidian pathogen isolated from Crepidodera aurata was identified based on morphological and ultrastructural characteristics, coupled with a molecular phylogenetic analysis. The spores of the microsporidian pathogen were slightly curved in shape, and measured 2.44-3.55 mu m in length and 1.25-1.55 mu m in width (n = 50). Its ultrastructure is characteristic of monokaryotic groups. All lifecycle stages of the pathogen, including meronts, sporonts, sporoblasts, and mature spores, are monokaryotic. The spore has 6-8 windings of the polar filament. Morphological and ultrastructural characteristics of the lifecycle stages place it within the family Unikaryonidae. However, the phylogenetic tree constructed on the 16S rRNA gene sequence analysis indicates that the pathogen is closely related to the Nosema/Vairimorpha Glade of microsporidia. Therefore, we have classified the microsporidian of C. aurata in the tentative group Microsporidium in order to avoid creating an unnecessary or incorrect new genus/species.