SYNOPSIS.Nosema lepidurin. sp. parasitic inLepidurus apusL. is described. The microsporidian invades the connective tissue cells of carapace, head and legs of the host crustacean. The developmental cycle and cytology of the organism were studied. The spores stained by McManus method show a small polar cap located at the more attenuated pole, a structure not hitherto reported. It is different from the polar granule which is commonly present in microsporidian spores. The polar cap stainable by McManus method is probably present in the spores of all Microsporidia, though its chemical nature is unknown.