Previous studies have shown that the organization cell wall microfibrils precisely mimics the pattern of cytoplasmic microtubules in developing cotton fibers. This relationship is evidenced from the early stages of fiber elongation (3 days post antheses [DPA]), well into secondary wall synthesis (36 DPA). Serial section reconstruction analysis at the electron microscopic level of in vitro grown fibers from Gossypium hirsutum (variety Acala SJ-2), indicates that arrays of microtubules are more ordered during the later stages of primary wall synthesis. The transition from primary to secondary wall synthesis is accompanied by increases in microtubule numbers and lengths. These data indicate that changes in microtubule arrays may be essential for proper wall development.