The band of external sulcus cells(ESCs) is one of the structures contained in the lateral wall of the scala media along with the spiral prominence and the stria vascularis, and is thought to play an important role in the metabolism of the endolymph. Thus a morphologic study of the lateral wall is necessary to understand the cochlear physiology and pathophysiology. Since 1869 when Boettcher first referred to these cells as those of the external sulcus or root cells, many studies have been performed. However, the exact morphology and function of the ESCs have not yet been determined. The purpose of this study is to 1) study the morphology of the ESCs using both light and electronmicroscope, and 2) demonstrate any morphologic differences among species(chinchillia, guinea pig, cat), and 3) propose the function of the ESCs. The results are as follows. 1) The external sulcus cells forms a continuous band which runs the length of the cochlear duct on the basilar membrane and spiral ligament between the spiral prominence and Claudius cells. 2) External sulcus cells have prominent nucleoli and stain more deeply than Claudius cells but less than the spiral prominence epithelium allowing them to be easily identified. 3) In the basal and middle turns of the cochlea, external sulcus cells are usually located below the Claudius cells and the spiral prominence epithelium, but in the third turn they are exposed to the endolymphatic space through gaps in the Claudius cells and spiral prominence epithelium. When external sulcus cells are exposed to the endolymphatic surface, they display microvilli on the exposed cell surface. 4) When viewed with a scanning electronmicroscope, the surface of the external sulcus cell is identical to that of the Claudius cell, and it is impossible to differentiate them. But spiral prominence epithelium is easily identified due to its rod like shape. 5) The external sulcus cell has long slender pegs or cords which extend upward and outward behind the stria vascularis and spiral prominence. Many capillaries accompanied by pericytes are distributed in the spiral prominence and surround the pegs and cords. 6) Many microvesicles suggestive of micropinocytic activity are found along the cell membrane of the ESCs. 7) Unusual membrane enclosed bodies(BEMs) which suggest phagocytosis are found within the external sulcus cells, as well as within the pericytes, and most of all in the extracellular space adjacent to the capillaries. The MEBs contain ground substance, microvesicles, and occasionally tangled membrane structures. 8) No significant differences in the fine structures of the external sulcus cells among the chinchilla, guinea pig or cat were noted. These observations indicated that the ESCs possess both structures related to pinocytosis or phagosytosis and structural characteristics compatable with material tansport. This suggests that the function of the external sulcus cells is transport from the scala media to the capillary rather than endolymph secretion and nourishment to the organ of Corti.