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Few studies of phenotypic selection have focused on physiological traits, especially in natural populations. The adaptive significance of plant water-use efficiency, the ratio of photosynthesis to water loss through transpiration, has rarely been examined. In this study, carbon isotopic discrimination, Delta, an integrated measure of water-use efficiency, was repeatedly measured in juveniles and adults in a natural population of the herbaceous desert perennial Cryptantha flava over a 4-year period and examined for plasticity in Delta, consistency between years in values of Delta, and evidence for selection on Delta phenotypes. There was significant concordance in Delta values among the 4 years for adult plants and significant correlations in Delta values measured in different years for juveniles and adults combined. The wettest year of the study, 1998, proved an exception because Delta values that year were not correlated with Delta values in any other year of the study. Consistency in Delta measured on the same plants in different years could indicate genotypic variation and/or consistency in the water status of the microhabitats the plants occupied. Two forms of plasticity in Delta were also evident; mean seasonal values were correlated with precipitation the preceding autumn, and Delta values also declined with plant size, indicating increasing water-use efficiency. Phenotypic selection was evident because in the first year of the study juvenile plants that would survive until year five averaged lower Delta values than did those that failed to survive. During the driest year, 2000, Delta was significantly negatively correlated with adult plant size, measured as the number of leaf rosettes, but the negative relationship between Delta and the number of flowering stalks, a more direct measure of fitness, was not significant. These results suggest that the direction of phenotypic selection on Delta changes as plants grow.