This quantitative study of the cellulose content of secondary walls of never dried cotton fibers uses a technique based on colorimetry. The study reveals that the density of cellulose, i.e., the mass of cellulose per volume of secondary wall, is identical for all never dried cotton fibers and independent of the genotype. Various plant growth conditions do not influence this cellulose density. For samples of never dried fibers with a given mean length, the mean secondary wall thickness is an accurate indicator of the amount of cellulose produced in these fibers.