Nosema sp. develops primarily in the fat body, causing darkening of the larvae and often death before or during the pupal period. Stages in the life history are described and the spore is interpreted as having the polar filament coiled in the peripheral region with its free end expanded to form a sac containing the sporoplasm. Observations on filament extrusion beyond suspended films onto glass indicate that pressure within the spore causes the eversion of a hollow filament through which the sporoplasm passes to appear at the tip. Penetrating ability of the filament and its firm attachment to the spore case were demonstrated by feeding spores to ciliates.