The ocelli of Periplaneta americana were studied by light and electron microscopy. The view that the ocellus of the cockroach represents a degenerated structure can no longer be supported. All organelles necessary for function are present. The club-shaped retinular cells lie homogeneously distributed in the cupule of the ocellus. Rhabdoms are seen as sickle-, y-, x- or star-shaped structures with up to six cells in formation. Cells were found which had formed two rhabdomeres. The mass of cell organelles lies in the cytoplasm between the cell nucleus and the rhabdom. Smooth endoplasmic reticulum is wound into a spindle formation of considerable size at the origin of the axon in some cells. A cylindrical body in which 10-40 microtubules are packed, as yet unknown in insect retinular cells, is described. The receptory area of the ocellus terminates in a tapetum which contains granules, soluble in alcohol. The axon bundles of the retinular cells run through the tapetum and immediately thereafter make synaptic contact with dendrites of the ocellar nerve cells, while still within the ocellus.