The development of lint fibers of field‐grown ‘Stoneville 213’ cotton (Gossypium hirsutum L.) was studied from anthesis to maturity with reference to lint fiber elongation, dry lint weight per seed, and dry lint weight per seed per unit of length. These variables were plotted against boll age and fitted to appropriate best‐fit curves by computer curvilinear regression analysis. The curve‐fitting procedures yielded mathematical equations which were differentiated to give accurate growth rate curves. The elongation rate curve showed that lint fiber elongation ceased at 27 days after anthesis. At that time, dry fiber weight and dry fiber weight per unit length had reached 40% of their final values. Splining analysis of the data shows that fiber dry weight per unit length ceased to be constant at 16 to 19 days after anthesis; theoretically, this is the age at which secondary wall thickening began. These data strongly suggests that the elongation and secondary wall thickening phases were not separated in time and that a substantial portion of secondary wall thickening occurred before fiber elongation ceased.