The objectives of this study were: (1) to quantify the genetic variation in foliar carbon isotope composition (d13C) of 122 clones of ca. 4-year-old F1 hybrids between slash pine (Pinus elliottii Engelm var. elliottii) and Caribbean pine (Pinus caribaea var. hondurensis Barr. et Golf.) grown at two field experimental sites with different water and nitrogen availability in southeast Queensland, Australia, in relation to tree growth and foliar nitrogen concentration (Nmass); and (2) to assess the potential of using d13C measurements, in the foliage materials collected from the clone hedges at nursery and the 4-year-old tree canopies in the field, as an indirect index of tree water use efficiency for selecting elite F1 hybrid pine clones with improved tree growth. There were significant differences in foliar d13C between the nursery hedges and the 4-year-old tree canopies in the field, between the summer and winter seasons, between the two experimental sites, and between the upper outer and lower outer canopy positions sampled. This indicates that d13C measurements in the foliage materials are significantly influenced by the sampling techniques and environmental conditions. Significant differences in foliar d13C, at the upper outer canopy in both field experiments in summer and winter, were detected between the clones, and between the female parents of the clones. Clone means of tree height at age ca. 3 years were positively related to those of the upper outer canopy d13C at both experimental sites in winter, but only for the wetter site in summer. There were positive, linear relationships between clone means of canopy d13C and those of canopy Nmass, indicating that canopy photosynthetic capacity might be an important factor regulating the clonal variation in canopy d13C. Significant correlations were found between clone means of canopy d13C at both experimental sites in summer and winter, and between those at the upper outer and lower outer canopy positions. Mean clone d13C for the nursery hedges was only positively related to mean clone stem diameter at 1.3 m height at age 3 years on the wetter site. The clone by site interaction for foliar d13C at the upper outer canopy was significant only in summer. Overall, the relatively high genetic variance components for foliar d13C and significant, positive correlations between clone means of foliar d13C and tree growth have highlighted the potential of using foliar d13C measurements for assisting in selection of the elite F1 hybrid pine clones with improved tree growth.