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The levels of tubulin protein in developing cotton (Gossypium hirsutum L. cv. Stoneville 825) fibers were measured from 8 to 28 days post‐anthesis using commercially available monoclonal antibodies against alpha‐ and beta‐tubulin. As the monoclonal antibodies against alpha‐ and beta‐tubulin were prepared from yeast tubulin and chick brain tubulin, respectively, indirect immunofluorescence microscopy was used to establish that the two monoclonal antibodies recognized microtubule structures in cotton fibers. Western blots of electrophoretically separated proteins in crude extracts of cotton roots and fibers showed that single polypeptides with the expected apparent molecular weight for tubulin subunits were recognized by the antisera. An enzyme‐linked immunosorbent assay was used to quantify tubulin levels. 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 correlated with the elongation of the fiber and an increase in cellulose synthesis. [ABSTRACT FROM AUTHOR]