Postembryonic development of the dorsal ocellus of the cockroach Periplaneta americana was examined. Small ocelli (20 micron in diameter) with less than 100 cells in the newly hatched nymph become adult ocelli (more than 500 micron in diameter) with more than 10,000 retinular cells, through ten to 11 nymphal stages. Thus, sequential steps of morphogenesis of rhabdomeres from loose interdigitations of apposed cell membranes to regularly arranged microvilli can be seen in the nymphal ocelli. Some retinular axons appear to extend into the brain as a bundle in the first-instar nymph. Retinular axons differentiated later also extend toward the brain, but there is no evidence that they enter it. The ultrastructure of the ocellus and component analysis of the ocellar electroretinogram suggest that functional connections between retinular axons and ocellar interneurons do not occur in the ocellar capsule until mid (the fifth or sixth)-instar nymphs. The ocellar diameter increases linearly with increase in body length during the nymphal stages, but it increases by a factor of 1.5-2.0 during the final molting: the body length of the adult is almost the same as that of the last-instar nymph. These data suggest that the function of the dorsal ocelli may be closely related to specific adult behavior such as flight.