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Carbon isotope discrimination is used increasingly in ecological research as an indicator of plant water use efficiency, although field sampling methods may not always consider the genetic and environmental variability, both spatial and temporal, which contribute to the foliar isotopic signature. In greenhouse manipulations of water availability to cloned female plants of Simmondsia chinensis, a dioecious Sonoran Desert shrub, foliar delta(13)C increases 2.3 parts per thousand with increased water deficit, while leaf area is reduced 4-fold. Long-term morphological adjustment may be more important than modification of water-use efficiency in response to water limitation in S. chinensis. In contrast with this experimental data, we found a 5.1 parts per thousand range in delta(13)C in a natural population of S. chinensis in an apparently homogeneous habitat, despite consistent within-plant values. delta(13)C values were correlated with leaf size, but not consistently with plant size, leaf specific mass, or foliage area density. delta(13)C was also correlated with the seasonal amplitude in predawn water potential. We found no other correlation of delta(13)C with field measures of plant water status, nor with instantaneous measures of water-use efficiency or gas exchange, most likely due to leaf age. There were no differences in carbon isotope ratios between female and male plants. We attribute this wide variability in isotopic composition in the field at least in part to genotypic diversity within the population of this dioecious species. Sampling technique for stable carbon isotope analysis in field studies of natural populations should consider the implications of potential genetic variability.