Cotton fiber cellulose synthesis during development and cellulose content in mature fibers were investigated by using 3H-labeling for 3 cultivated cotton species, G. hirsutum, G. barbadense, and G. arboreum. There was no significant correlation between fiber strength and cellulose content for the mature fiber of different species or varieties if cellulose content was higher than 80%. However, if cellulose content was lower than 80% for the same variety, there was a significant correlation between fiber strength and cellulose content. Therefore, for any one variety, increasing the cellulose content is valuable to enhance fiber strength and micronaire. In contrast, the mean rate of cellulose deposition had no significant influence on fiber strength, whereas dynamic changes in its deposition rate did have an effect. The species (G. barbadense) or variety (Xu-576) with higher fiber strength had only deposited less than 50% of their final cellulose content before 25 DPA, and they kept a higher stable deposition rate from 25 to 45 DPA. Other species (G. hirsutum and G. arboreum ) or varieties (LM-1 and LM-6) had deposited nearly 70% of their final cellulose content by 25 DPA. Fiber strength is enhanced by high-rate cellulose deposition over a longer period as long as late-season growing conditions are not stressful. In contrast, stability of fiber strength and micronaire between growing seasons is enhanced in species or varieties with early-season, high-rate cellulose deposition when growing conditions are less likely to be stressful.