Three-dimensional structures of the capitulum and first mitochondria in the neck region of the hamster spermatozoa, were observed with transmission (TEM) and scanning electron microscope (SEM). Some capitula revealed a variable contour even in the caudal epididymis, but most finally developed to form a typical wagonette shape. The final shape of the capitulum is probably produced by the aid of apical protrusion of the right, pyramidal mitochondrium. The right and left first mitochondria were triangular pyramids in contour, while the dorsal and ventral ones were rod-like in shape. The mutual transformations between the capitulum and the first mitochondria are discussed in relation to the completion of the neck region.