The crystalline structure of dried cotton fibers at varying development stages has been investigated using wide angle x-ray diffraction (WAXS) techniques. The cellulose I crystalline structure has been confirmed on dried SJ-2 Acala. cotton fibers collected at varying developmental stages and at maturity. The cellulose I crystalline structure is clearly evident at the early developmental stage of 21 days postanthesis (dpa). The crystal system remains unchanged during the cotton fiber biosynthesis and at maturity. The degree of crystallinity and crystallite dimensions in the cotton fibers increase with cell development. The most significant increments are observed between 21 and 34 dpa (i.e., during the first half of the secondary wall thickening process). The unit cell sizes slightly decrease and thus the crystal density increases with fiber development. The alignment of the glucosidic rings in respect to the 002 planes improves with fiber cell development.