Cotton fiber chronological maturity is estimated by days post floral anthesis [DPA], but cotton classing and processing qualities depend on physical fiber maturity [ratio of fibers of acceptable wall thickness]. Physiologically, maturing cotton fibers thicken as cellulose is deposited in the secondary wall, a process that results in dilution by weight of primary wall components [pectins and associated calcium]. Fiber chronological, physical and physiological maturation rates were compared when bolls from field-grown Upland and Pima varieties were harvested from 21 to 56 DPA. Fiber physical maturities were assayed at the boll-level by AFIS-F&M. Fiber physiological/biochemical maturities were determined by x-ray fluorescence [Ca-XRF] as the weight ratio of calcium-rich primary cell wall to low-calcium secondary wall. Fiber maturities and maturation rates were species- and genotype-specific, regardless of maturity measurement methodology. Growth environment [crop year, flowering date, temperature, and field or greenhouse] significantly altered maturation rates.