On each lateral side of the cephalothorax segments, the adult Sinelobus stanfordi has a branchial chamber which contains an elongated bag-shaped gill and is covered by a thick branchiostegite. The ultrastructural study revealed that the inner surface of the branchiostegite is composed of a transporting-type epithelium which is morphologically distinct from the gill epithelium. Both epithelia are covered by extremely thin (about 80 nm) cuticle layers, suggesting high permeability to gases, ions, and water. In contrast, the outer surface of the branchiostegite consists of ordinary epithelium covered by a very thick cuticle layer in common with the body surface. The inner branchiostegal epithelium (4–10 μm thick) has a shallow (about 1 μm deep) apical infolding system of the cell membrane (AIS) and an extensive three-dimensional tubular network (about 120 nm in diameter) which is formed by the invagination of the basolateral cell membrane (TNB). The TNB is associated with slender mitochondria and occupies the majority of the cytoplasmic area of the epithelial cell. The gill epithelium, on the other hand, is about 10 μm thick and characterized by an abundance of oval mitochondria, well-developed (4–5 μm deep) AIS, and a smooth basal cell membrane lacking any infoldings. These morphological features indicate that not only the gill epithelia, but also the inner branchiostegal epithelia, are involved in the ion-transporting processes. The ultrastructural differences between these two kinds of epithelia also suggest their different roles in the osmoregulation of this animal, since it inhabits estuaries which are subject to extreme changes in salinity.