Abstract Photosynthesis is the process that cotton plants use to fix carbon from the atmosphere to generate new growth and ultimately, yield. Attempts to increase genetically the rate of photosynthesis requires a knowledge of the relationship between photosynthesis and growth stage, fruit load and environmental parameters. The purpose of this research was to determine if genetic differences in sink-to-source ratios influenced single-leaf photosynthetic rates. Five cotton ( Gossypium hirsutum L.) genotypes differing in sink-to-source ratios were grown in a field environment. Photosynthetic rate was estimated at mid-morning and mid-afternoon with 14CO 2 gas exchange on the uppermost fully expanded mainstem leaf. Seven measurements of photosynthetic rate were made at intervals commencing 52 days after planting (DAP) and terminating 104 DAP. At each sampling date solar irradiance, leaf and air temperature and leaf conductance were also measured. Correlations between these parameters and photosynthetic rate at each sampling date and over the entire season were generally not statistically significant. The occasional significant correlations were small. After photosynthetic rate was estimated, plants were harvested; leaf areas and dry weights were then measured. Photosynthetic rate, measured on single leaves, varied in response to light, leaf temperature, leaf conductance, growth traits and sink-to-source ratios (weight of fruit and stem per unit leaf area). Photosynthetic rate was low at the first sampling date and increased thereafter with significant differences being expressed among the five genotypes. Sink-to-source ratios increased at each sampling date with significant differences among genotypes. Single-leaf photosynthetic rate increased as the sink-to-source ratios increased.