Microtubules are organelles involved in the deposition of cellulose in cell walls. The protein tubulin is the major component of the microtubule. The levels of tubulin protein in developing cotton (Gossypium hirsutum L.) fibers were measured from 8 to 28 days post-anthesis using commercially available monoclonal antibodies against alpha- and beta-tubulin. From 10 to 20 days post-anthesis the level of tubulin protein increases approximately three-fold. After 20 days post-anthesis, the amount of tubulin relative to total fiber protein reaches a plateau or decreases slightly. The rapid rise in tubulin is positively correlated with the elongation of the fiber and an increase in cellulose synthesis.