Classical quantitative genetics and quantitative trait dissection analysis (QTL) approaches were used in order to investigate the genetic determinism of wood cellulose carbon isotope composition (δ13C, a time integrated estimate of water use efficiency) and of diameter growth and their relationship on adult trees (15 years) of a forest tree species (maritime pine). A half diallel experimental set-up was used to (1) estimate heritabilities for δ13C and ring width and (2) to decompose the phenotypic δ13C/growth correlation into its genetic and environmental components. Considerable variation was found for δ13C (range of over 3‰) and for ring width (range of over 5 mm) and significant heritabilities (narrow sense 0·17/0·19 for δ13C and ring width, respectively, 100% additivity). The significant phenotypic correlation between δ13C and ring width was not determined by the genetic component, but was attributable to environmental components. Using a genetic linkage map of a full-sib family, four significant and four suggestive QTLs were detected for δ13C, the first for δ13C in a forest tree species, as far as known to the authors. Two significant and four suggestive QTLs were found for ring width. No co-location of QTLs was found between δ13C and growth.