Three types of mature epidermal neurons and several of their differentiating stages aie presented in this ultrastructural study. Each of the three types, neurosensory, neurosecretory, and ganglionic cells, is derived from interstitial cells, (i) Mature neurosensory cells contain elongated nuclei, a well-developed cilium in each cell, and membrane-bounded neurosecretory droplets (700–1300 A in diameter). There may be two or more neurites in which are numerous microtubules, glycogen particles, ribosomes and many neurosecretory droplets, (ii) Mature neurosecretory cells closely resemble neurosensory cells, except that no cilium is present. The perikarya contain small, dense nuclei, neurosecretory droplets (850–1300 A in diameter), mitochondria, glycogen particles, and microtubules. Active Golgi complexes are present in both cell types. The nemites are similar to those described for neurosensory cells, (iii) Mature ganglionic cells are bipolar or multipolar. The small, dense nuclei are surrounded by a small amount of cytoplasm. The neurites contain mostly microtubules; a few mitochondria, ribosomes, and glycogen particles are also present, but there are no secretory droplets. To date, only neurosensory and neurosecretory cells have been observed in the gastiodermis. They are structurally indistinguishable from their epideimal counterparts. A significant finding is that three types of synapses—neuromuscular, neuronematocyte, and interneuronal—are identified in both the epidermal and gastrodermal neurons.