The subcellular structure of epididymal spermatozoa obtained from standard dark mink was studied by means of electron microscopy, using thin-sectioning techniques. The spermatozoan head was seen to be dorsoventrally flattened and ovate in outline. The anterior two-third of the nucleus was covered with the acrosome which can be divided into three segments (apical, man and equatorial) according to the acrosomal content. The posterior one-third was covered with the postaucrosomal sheath which was a dense layer deposited on the inner aspect of the cell membrane. On the dorsal and ventral and ventral and ventral aspects of the bead, six swellings were observed; two of these were located at each side of the proximal border of the equatorial segment, and one was associated with the pestacrosomal region on each side. The cell membrane adhered firmly to the tip of the acrosome and to the postacrosomal sheath, but was found to be usually separated from the rest of the acroseme. The articular structure of the neck appeared to show separate dorsal and ventral plates of the capitulum which were followed by a ring of striated columns of the connecting piece. These striated columns were followed by two major and five minor columns which appeared io continue with the nine dense fibers of the axial fiber bundles. Beneath the inner surface of the capitulum, the proximal centriole wes found in the center of the connected ring. The spermatozoan tail displayed a 9+9+2 pattern in the organization of the axial fiber bundle consting of the dense fibers, double microtubules, and a central pair. The dense fibers number 9, 1, 5, and 6 were larger in diameter than the rest of the dense fibers. In the axonemal complex, the diameter of subfiber A was larger than the central fiber, while that of subfiber B was the smallest. The middle piece was of medium length compared with other mammalian spermatozoa. The annulus was triangular in longitudinal sections and contained nonhomogeneously distributed electron dense material. The fibrous sheath had longitudinal columns. Although the end piece maintained the 9+2 pattern, it did not have the fibrous sheath.