In the cytology of the developing rabbit spermatid two systems of microtubules are conspicuous features. One corresponding to the manchette of light microscopy is in correlation with changes in the nuclear membrane giving rise to the posterior nuclear space. This system disappears as the elongation of the nucleus is accomplished without leaving any remnants. In the late spermatid another system of microtubules is observed just anterior to the fibrous sheath inside the periflagellar invagination giving this part of the flagellum a spindle shaped form. — The possible functions of the microtubules and related cellular events are discussed.