Specimens of the ductus deferens from normal adult men of proven fertility were examined with the light and electron microscope. The wall of the ductus deferens is made up of a mucosa, three concentric layers of smooth muscle and an adventitia. The epithelium, together with the underlying connective tissue of the lamina propria, is thrown up into longitudinal folds. The pseudostratified epithelium covering the folds is composed of small basal cells which form a discontinuous layer along the basement membrane and tall thin columnar cells which extend from the base of the epithelium to the lumen. In addition to basal cells, at least three different types of columnar cells can be distinguished in the electron microscope: principal cells, pencil cells and mitochondrion-rich cells. Lymphocytes are found scattered amongst the epithelial cells. The morphological complexity of the epithelium of the ductus deferens suggests that the epithelial cells of this region of the genital ducts, like those of more proximal regions, perform functions of importance to the spermatozoa.