The fine structure of epididymal spermatozoa of Myotis lucifugus and Eptesicus fuscus is described. Caudal to the acrosomal cap in both species, a thin intermediate layer is described as uniting the plasmalemma to the outer leaf of the nuclear envelope. Although this layer does not form a separate cap-like structure, it evidently corresponds to the “post nuclear cap” of light microscopists. A redundant portion of the nuclear envelope turns away from the condensed chromatin and continues caudad into the neck region where it forms a pair of concentric membrane systems on either side of the connecting piece. Two flattened mitochondria are closely applied to the dorsal and ventral aspects of the connecting piece.

The sperm tails in the two species are generally similar except for the shape and distribution of mitochondria in the middle-piece and the thickness of the fibrous sheath of the principal piece. A unique arrangement of the mitochondria in Myotis makes it possible to determine the orientation of the plane of the central pair of flagellar fibrils in relation to the transverse axis of the flattened head and hence to deduce the probable plane of bending movments of the tail. Cortical and medullary zones are distinguished in the outer dense fibers of the flagellum and fibers 9, 1, 5, and 6 are consistently larger than the others. A set of slender satellite fibers are closely related to the inner aspect of the nine large outer fibers. Additional structural details of the axial filament complex are described, especially the cross-sectional configuration of the subfibrils in the doublets and their mode of termination in the end-piece. No “secondary fibers” or “mid fiber” were found.

Since the axial filament complex of mammalian sperm tails is not continuous with a basal body, as in other flagella, it is suggested that the connecting piece is a centriolar derivative and may be the functional equivalent of a basal body for both the axial complex and the outer dense fibers.

Comparisons are made of bat sperm tails and those of other mammalian species and the ultrastructure and arrangements of tail components are discussed in relation to the current concepts of sperm locomotion.