Water use efficiency, the amount of carbon acquired by a plant per unit of water lost, can be estimated by determination of the carbon stable isotopic composition (d13C) of plant material. d13C is a time integrated estimator for intrinsic water use efficiency (WUEi) which can be measured on an instantaneous basis as the ratio of CO2 assimilation rate over stomatal conductance for water vapour (A/g). The large variation detected for d13C within populations of forest tree species incited the research for a genetic basis for this variation. To this end we measured the variation of this complex quantitative trait within a full-sib pedunculate oak (Quercus robur L.) family to decompose it into mendelian-inherited genetic components (QTL analysis). We were also interested in the comparison of the within family variation of d13C with the variation of the ecophysiological components of water use efficiency: stomatal conductance, assimilation rate and other estimators of photosynthetic capacity. Two plantations of cuttings from the original pedunculate oak cross were available for harvesting of leaves. The first plantation, consisting of 174 siblings with on average five vegetative copies per sibling, was sampled in 2000 for d13C and leaf nitrogen analysis and a subsample was chosen in 2001 for gas exchange analysis. The second plantation with a larger number of siblings than the first one (207) was sampled in 2002 for d13C and leaf nitrogen analysis. Leaves were collected in 2002 from the first plantation and were analysed for d13C and leaf nitrogen. QTL analysis was performed taking account of clone and bloc effects and resulted in the detection of two highly significant QTLs for d13C and two highly significant QTLs for leaf nitrogen, each QTL explaining more than 15% of the detected phenotypic variance. One QTL for d13C and one QTL for leaf nitrogen were co-located in the same interval on the same linkage group. The gas exchange study in 2001 on 120 siblings from the same plantation was carried out over three days to investigate the dependence of leaf d13C on assimilation rate and stomatal conductance. Mean sibling leaf d13C as measured in 2000 correlated significantly with the mean sibling leaf d13C as measured in 2001 (r2 = 39%). A weak but significant relationship between leaf d13C as measured in 2001 and A/g was detected. The trend of higher A/g with less negative leaf d13C is in accordance with theoretical model. Stronger relationships were observed between leaf d13C and leaf nitrogen per leaf area or leaf chlorophyll content, two traits related to photosynthetic capacity. This is in concordance with the detected co-localisation of a QTL for d13C with a QTL for foliar nitrogen in 2000. In 2002, the second plantation of the same pedunculate oak cross was used for foliar d13C and nitrogen analysis. QTL analysis will be performed for leaf d13C and foliar nitrogen concentration to validate the strong QTLs found in 2000 for these traits under different environmental conditions and covering a larger number of siblings.