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The basis used for expression of carbon dioxide exchange rate (CER) of single leaves may alter the relative ranking of genotypes being selected for a breeding program. Therefore, field and growth chamber experiments were conducted on 16 genotypes of tall fescue (Festuca arundinacea Schreb.) to determine the relative merit of expressing CER data on a leaf area or leaf weight basis. Genotypes were grown in plots in the field and in pots in the growth chamber. The CER was measured using an infrared gas analyzer. Herbage yield was determined. Significant genotypic variation within experiments was found for CER when expressed on either a leaf area or leaf weight basis. The correlation coefficient between CER on a leaf area and leaf weight basis ranged from 0.82 to 0.94 for the field experiments and was 0.76 for the growth chamber experiment. Comparison of coefficients of variation (CV's) associated with the two methods of expressing CER indicated slightly higher variation in data based on leaf weight. The CV's for the field experiments were between 11.5 and 16.5%, suggesting that either method of expression of CER for tall fescue could be satisfactorily used to detect genotypic differences. Leaf area is suggested as the preferred base because of its association with radiation interception. Further, genetic control of CER per unit leaf area appears to be independent from specific leaf weight.