Carbon isotope discrimination ( ) has been used to identify plants with differences in water use efficiency for a number of important C(3) field crops. Experiments were conducted at Temple, College Station, and Lubbock, TX in 1992 to evaluate the use of in identifying high yielding cottons. Data indicated that significant differences in and yield were present among the 10 commercial varieties and 15 experimental strains studied. Yield and of well watered cotton plants were highly correlated when was obtained from leaves of young plants at first fruit appearance. Although our results warrant further examination and study, these data suggest that may be a good indicator of potential lint yield in cotton. This technique may enable cotton breeders to screen and identify breeding strains that have superior photosynthetic capacity early in the breeding cycle, thereby speeding the development of high yielding cottons.