The lateral wall of the dog cochlear duct was investigated by classical staining and immunohistochemistry for NaK/ATPase β2 isoform, cytokeratins (Cks), vimentin, nestin, and S100A6 during the postnatal cochlear maturation, i.e., from birth to postnatal day 110. The dog stria vascularis was immature at birth. Fine melanin granules were evident in the stria from the second week of life, and melanin concentration increased drastically beyond the first month. The marginal cells were NaK/ATPase- and Ck-positive; intermediate cells were either nestin- and S100A6-positive or vimentin-positive; the basal cells were vimentin-positive; the capillary endothelium showed vimentin and nestin labeling; the cell layer underlying the stria was nestin-positive. The fibrocytes of the spiral ligament and spiral prominence expressed nestin and vimentin. The epithelial cells overlaying the spiral prominence and the external sulcus were Ck-positive, and transiently nestin- and vimentin-positive. Double immunolabeling, for S100A6 and either nestin, vimentin, or NaK/ATPase, and for nestin and vimentin suggested the presence of two distinct intermediate cell types. The results enabled us to differentiate the cell types forming the lateral wall of the dog cochlear duct, and to follow their postnatal maturation. This study may form a basis for future investigations about spontaneous cochleosaccular degeneration in dogs. This species is an important companion animal, and a possible model for the study of comparable diseases in humans. Anat Rec Part A 270A:82–92, 2003. © 2003 Wiley-Liss, Inc.