Neurons in the median visual system of the barnacle were injected with horseradish peroxidase (HRP) and their processes traced in serial thick (3 micrometers) and thin (100 nm) sections with light and electron microscopy. The anatomy confirms that the I-cells identified by Oertel and Stuart (Oertel, D., and A. E. Stuart (1981) J. Physiol. (Lond.) 311: 127-146) are second-order neurons in the median visual pathway. The terminal branches of the photoreceptor axons are associated in each half of the supraesophageal ganglion with a region of neuropil that extends for 30 to 40 micrometers along the commissure; this photoreceptor-associated neuropil is continuous with but much simpler than the main mass of neuropil in the core of each hemiganglion. The photoreceptor-associated neuropil consists of a small number of different neuronal elements, each with distinctive cytological features. The photoreceptor terminal processes dominate the neuropil; in addition, one finds there the most medial processes of the arborizations of the ipsilateral and contralateral I-cells and the processes of an axon or class of axons of unknown origin referred to as the "RL" fibers. These RL processes associate and branch with the receptor axons and constitute the only major neuronal element in this region of neuropil besides the median photoreceptors and the I-cells. In preparations with an HRP-filled I-cell, the photoreceptors were shown to be presynaptic to I-cell processes at synapses which are characterized by a pair of closely apposed postsynaptic profiles referred to as a dyad. The filled I-cell constituted only one element of the dyad. Receptor endings were also presynaptic to dyads where both postsynaptic elements were unfilled. The RL processes were presynaptic to the photoreceptor terminals at nondyadic synapses, and the receptor endings made specialized junctions, possibly synaptic, with the RL processes. Lateral to the region occupied by the photoreceptor arborization, varicosities of the I-cell are juxtaposed to processes of the A-cell; thus, these sites of the I-cell, where it is most likely to be presynaptic to the A-cell, are segregated from the sites at which it is postsynaptic to the photoreceptors. The I-cell is postsynaptic to photoreceptors and has varicosities juxtaposed to A-cell processes in both hemiganglia.