Cubomedusae, or box jellyfish, have the most elaborate visual system of all cnidarians. They have 24 eyes of four morphological types, distributed on four sensory structures called rhopalia. Box jellyfish also display complex, probably visually guided behaviors such as obstacle avoidance and fast directional swimming. Here we describe the strikingly complex and partially bilaterally symmetrical nervous system found in each rhopalium of the box jellyfish, Tripedalia cystophora, and present the rhopalial neuroanatomy in an atlas-like series of drawings. Discrete populations of neurons and commissures connecting the left and the right side along with two populations of nonneuronal cells were visualized using several different histochemical staining techniques and electron microscopy. The number of rhopalial nerve cells and their overall arrangement indicates that visual processing and integration at least partly happen within the rhopalia. The larger of the two nonneuronal cell populations comprises 2,000 likely undifferentiated cells and may support a rapid cell turnover in the rhopalial nervous system.