The treatment with ferriosmium of meronts and sporonts of the microsporidian Enterocytozoon bieneusi has shown the lamellar organization of the membranes of the endoplasmic reticulum and of the nuclear envelope. The Electron Spectroscopy Imaging (ESI), (Ottensmeyer and Andrew 1980) of unstained ultrathin sections of meronts showed the presence of phosphorus in the lamellar structures thus confirming their phospholipidic composition. Phosphorus was also detected in the lamellar polaroplast of the spore.