AbstractSpecific leaf area (SLA), the ratio of leaf area to leaf dry weight, is used in crop simulation models to estimate total leaf area or dry weight. This experiment was designed to study changes in SLA in various parts of a cotton (Gossypium hirsutum L.) canopy throughout the growing season. The main stem of the cotton plant was divided into five‐node segments. Leaf area and leaf dry weight were measured for each segment throughout the growing season. The mean seasonal SLAs for the segments from the bottom to the top of the canopy were 26.2, 25.6, 20.9, 19.4, and 18.1 m2 kg−1. Except for the uppermost segment, SLA increased from 43 to 90 d after emergence (DAE) and declined from 100 DAE. The decline coincided with boll maturation but also with canopy defoliation. It was possible to account for 93% of the variation in SLA for all segments by plotting SLA against light flux density within the cotton canopy. Variations in SLA with time, growth stage, and leaf maturity were small to insignificant when variation due to light flux density was removed. Sampling for SLA should include leaves throughout the crop canopy. Modelers who use SLA to estimate total leaf area or dry weight should determine the relationship between SLA and light flux density for their particular crop.