The fine structure of ROHDE neurons of the amphioxus spinal cord has been studied using transmission electron microscopy. ROHDE cells are segmentary giant multipolar neurons with a large nucleus and nucleolus. The cytoplasm has a large amount of neurofilaments and numerous mitochondria, dictyosomes, and other organula. The ROHDE cells cross the central canal and send to it cilia and numerous microvilli. The cell surface is mostly covered by glial laminar processes which can also accompain to deep invaginations of the neuron plasma membrane. Chemical synapses were not found over the cell perikaryon or their dendrites, but contacts resembling "gap junctions" were common. Giant axons of ROHDE cells have large amounts of neurofilaments and are ensheathed by glial processes. ROHDE axons make small "en passant" synapses with other neuronal processes. The functional and evolutionary significance of these findings is discussed.