Ultrastructural localization of the phosphatase component of Na-K-ATPase in the inner ear of the guinea pig was studied utilizing p-nitrophenyl phosphate (NPP) as substrate. In the stria vascularis NPPase activity was restricted to the cytoplasmatic side of the basal plasma membranes of the marginal cells. In the spiral prominence enzyme activity was resolved only occasionally on the basolateral membranes of the spiral prominence epithelium, especially after longer incubation periods. The remaining inner ear tissue was unreactive. Possible transport mechanisms of potassium across the apical marginal cell membrane are discussed.