This study aimed at characterizing the behavior of two commercial clones of Eucalyptus grandis x Eucalyptus urophylla with photosynthetic active radiation (Qleaf), atmospheric vapor pressure deficit (VPD) and predawn leaf water potential (Psi(pd)). The study was conducted in an experimental plot of the Faculty of Agricultural Engineering, State University of Campinas (FEAGRI / UNICAMP), located in Campinas, Sao Paulo, Brazil. The clones were of the same age (240 days) and at the same site of planting. Measurements of transpiration (E), stomata conductance (Gs) and photosynthesis (A) were carried out from January to July 2008 with the help of a porometer and Psi(pd) measurements were performed with the aid of a Scholander pressure chamber. Differences were observed in the responses of clones to environmental variables as well as in water use efficiency (WUE). The information gained may be used for better adjustment of forest management plans according to the ecophysiological specificity of each clone, for hydrological aspects of a watershed.3992419431