An X-ray diffraction and polarization-optical study of the cell wall of developing fibers of the shoot of Asparagus officinalis L. has been made. The cellulose micelles are randomly arranged in very young cells but assume a statistically more transverse position during elongation. The micelles of the secondary wall are deposited in a helical angle of about 48-51° to the long axis of the cell. Although the degree of crystallinity remains constant at 55-60 % during all stages of development, micellar size appears to increase slowly during elongation and markedly during secondary wall formation. The significance of micellar size determinations and crystallinity interpretations is discussed.