This study evaluates the effect of water availability on tolerance and resistance to the aphid Chaitophorus leucomelas by clones of the double hybrid [(Populus trichocarpa x P. maximowiczii Henry) x (P. trichocarpa x P. maximowiczii)] (TM x TM), a hybrid that was previously described as resistant to this aphid. In a 2 x 2 experimental design implemented in a nursery, we were able to assess growth (branch length, number of leaves and branch base diameter) in saplings reared in the natural presence of aphids (natural aphid damage) and aphid-controlled conditions (undamaged) under both well-watered and drought stress conditions. We found that resistance was reduced under drought stress conditions, while tolerance in branch length was increased. Cost of resistance was detected as clones displaying higher tolerance grew less in the absence of aphids, whereas no evidence of costs associated with tolerance was found in any of the growth traits measured, A genetic trade-off between tolerance and resistance was detected, but this trade-off was not affected by water availability. Considering the average response of both defence strategies, well-watered trees seem to allocate more resources to resistance than to tolerance, whereas drought-stressed trees allocate more to tolerance than to resistance. This suggests that tolerance would imply a lower cost than resistance, and the shift to either strategy could be modulated by resource availability.We would like to thanks to Marta Albornoz for technical support during water treatments and two anonymous reviewers for helpful comments on earlier versions of the article. We also gratefully acknowledge funding from the Fondo Nacional de Desarrollo Cientifico y Tecnologico FONDECYT 1040675, Universidad de Talca through the Poplar Research Center, Fondo de Fomento al Desarrollo Cientifico y Tecnologico FONDEF D01I1131 and Anillo Programa Bicentenario de Ciencia y Tecnologia ACT38.5FONDEFclramirez@utalca.clFondo Nacional de Desarrollo Cientifico y Tecnologico FONDECYT [1040675, D01I1131]; Universidad de Talca; Anillo Programa Bicentenario de Ciencia y Tecnologia [ACT38]4FONDEF24