A new microsporidian species,Globosporidium parameciigen. nov., sp. nov., fromParamecium primaureliais described on the basis of morphology, fine structure, and SSU rRNA gene sequence. This is the first case of microsporidiosis inParameciumreported so far. All observed stages of the life cycle are monokaryotic. The parasites develop in the cytoplasm, at least some part of the population in endoplasmic reticulum and its derivates. Meronts divide by binary fission. Sporogonial plasmodium divides by rosette-like budding. Early sporoblasts demonstrate a well-developed exospore forming blister-like structures. Spores with distinctive spherical shape are dimorphic in size (3.7 +/- 0.2 and 1.9 +/- 0.2 mu m). Both types of spores are characterized by a thin endospore, a short isofilar polar tube making one incomplete coil, a bipartite polaroplast, and a large posterior vacuole. Experimental infection was successful for 5 of 10 tested strains of theParamecium aureliaspecies complex. All susceptible strains belong to closely relatedP. primaureliaandP. pentaureliaspecies. Phylogenetic analysis placed the new species in the Clade 4 of Microsporidia and revealed its close relationship toEuplotespora binucleata(a microsporidium from the ciliateEuplotes woodruffi), toHelmichia lacustrisandMrazekia macrocyclopis, microsporidia from aquatic invertebrates.