Water use efficiency (WUE, the ratio of dry matter produced to water used in transpiration) was studied in 17 poplar clones and hybrids. Although WUE could not be predicted from the poplar group alone, water efficiency was found to vary in different poplar genotypes. For example, a balsam poplar, Populus maximowiczii M-4 (section Tacamahaca), and a white poplar clone, P. alba A-499 (section Leuce), had twice the dry matter production per unit of water transpired compared with another clone of P. maximowiczii (M-13) and a Cottonwood clone, P. nigra N-80 (section Aegeiros), which showed low WUE. The reduced transpiration in water-efficient clones per unit of dry matter produced was associated with a higher stomatal resistance on the abaxial leaf surface. However, the physiological and morphological basis of WUE varied in different clones. Water-efficient clones exhibited one or more of the following adaptations restricting water loss: (i) conspicuous cuticular ledges or hairs above the pore openings of stomata, (ii) earlier partial stomatal opening in the morning, and (iii) smaller stomata and a lower stomatal frequency on the adaxial surface of the upper leaves. Poplar clones of low WUE exhibited less stomatal control of transpiration as a result of lower stomatal resistances and they lacked the above adaptations. Relative ranking of genotypes could not be predicted from either dry matter productivity or transpiration rate alone although the most water efficient clones generally exhibited lower transpiration rates compared with less efficient clones.