There is a 2-fold variation in the DNA content in the diploid species of Gossypium and between the diploid and tetraploid species. CER ranged from 16 in the D genome to 41 mg CO₂ dm^-2hr^-1 in the AD genome. Levels of "activated" RuBPCase (Rubisco) varied 2-3 fold from 39 in the D genome to 99 mg CO₂ dm^-2hr^-1 in the AD tetraploid genome, and from 39 in the D genome, and from 16 in the D genome to 70 mg CO₂ dm^-2hr^-1 in the C genome of the diploid species. The correlation coefficient of CER: Rubisco is r=0.86, P=0.06 and Rubisco:DNA is r=0.92, P=0.02. Leaf anatomy studies show that within the Gossypium species leaf cell size varies with DNA content and CER is related to leaf cell size and leaf thickness. The data support the conclusion that the rates of CER and the levels of Rubisco in the diploid and tetraploid species of Gossypium vary with DNA content.