Technical Abstract: Water-use efficiency (WUE) is an important criterion in the selection of grasses for irrigated pastures in semiarid regions of western North America. Carbon isotope discrimination has been proposed as an indirect criterion for evaluating WUE in cool-season grasses. Ten cultivars of tall fescue (Festuca arundinaceae Schreb.) were established in the field under a line-source irrigation system. Objectives of our two-year study were to evaluate: (i) trends in carbon isotope discrimination across a gradient of four water levels (WL-2, wettest to WL-5, driest) and (ii) relationships between carbon isotope discrimination and dry matter yield (DMY) within and across water levels. Consistent and significant differences (P < 0.05) were found among cultivars for carbon isotope discrimination when evaluated across two harvests at WL-2. Cultivars also differed significantly for carbon isotope discrimination (P < 0.01) across four water levels when evaluated at the later harvest date. As with harvest dates, relative differences among cultivars were consistent across water levels. Although a curvilinear trend was evident for some cultivars, mean carbon isotope discrimination values decreased in a near linear manner from WL-2 to WL-5. Based on orthogonal polynomials the mean trend across water levels was partitioned as 95% linear and 5% quadratic. Correlations between carbon isotope discrimination and DMY were nonsignificant at the wetter water levels (WL-2 and 3), but the correlations were positive at the drier levels (WL-4 and 5). We conclude that tall fescue cultivars can be selected effectively for WUE on the basis of carbon isotope discrimination averaged across sampling dates.