The ultrastructure of the crayfish segmental giant (SG) neuron is described, and compared to other identified and unidentified crayfish neurons. The SG was specifically stained by intracellular injection of horseradish peroxidase and is divided into four regions of interest. In the dorsal region, finger-like dendrites of the SG make contact with the through-conducting giant fibres (GF). These contacts are physiologically defined rectifying electrical synapses. They are characterized by the presence of 30-95 nm agranular vesicles in the presynaptic GFs, some postsynaptic density in the SG, and a narrowing of the intermembrane cleft to approximately 5 nm. There is little evidence for connecting cytoplasmic bridges. Unidentified neurons make chemical input with either round or elliptical vesicle types onto SG bottlenecks close to the electrical synapses. Ventral to the GFs, dendritic profiles of the SG make three sorts of contact with unidentified neurons. (a) Regions of close membrane apposition (approximately 5 nm) are presumed to be electrical output synapses, but there are no vesicles such as at the input synapses, and, again, little sign of connecting bridges. (b) Chemical input is received from unidentified presynaptic neurons containing either round or elliptical vesicles. These synapses are characterized by 30-75 nm presynaptic agranular vesicles, widened cleft (approximately 20 nm), granular cleft material and postsynaptic density. There is no sign of any presynaptic density. (c) Very occasional SG profiles containing vesicles and making output synapses to unidentified neurons occur. In the lateral neuropil at the edge of the ganglion the SG gives rise to a small tuft of very fine dendrites. These are nearly all laden with vesicles and ramify in a complex region of neuropil containing many small profiles which are also vesicle-laden. The SG axon diminishes in diameter as it progresses along its peripheral nerve root, and finally terminates at a blind ending near the base of the swimmerets. It is sheathed along its entire length, and there is no sign of vesicles within it. We conclude that the SG axon makes no peripheral output.