The morphogenesis of the fibrous sheath in the rat spermatid has been studied by electron microscopy and by EM radioautogaphy following injection of 3H-proline. The appearance of the developing fibrous sheath from the distal to proximal end of the principal piece has been examined in spermatids in each of the 19 steps of spermiogenesis. The precise timing of the various steps of its formation has been established and related to radioautographic observations on the synthesis and incorporation of proteins into the developing fibrous sheath. The longitudinal columns form slowly over a period of 15 days, appearing first at the distal end of the principal piece in step 2 and gradually extending in a proximal direction, ending at the level of the annulus in step 17. Throughout this 15-day period, proline-containing proteins are synthesized and incorporated into the growing columns. All of the ribs, on the other hand, arise from anlagen which are assembled along the length of the principal piece during a much shorter (4.5-day) period between steps 11 and 15 of spermiogenesis. New rib anlagen seemingly originate from bundles of proteinaceous filamentous material which are synthesized in the cytoplasm of step 11-15 spermatids and become aligned along the plasma membrane of the principal piece, starting at the distal end. These observations suggest that the two components of the fibrous sheath are assembled by means of two independent mechanisms that proceed asynchronously except during an overlap period of 2.5 days between steps 12 and 14.