To better understand the variability in mechanical properties caused by genetic differences in hybrid poplars, modulus of elasticity and modulus of rupture in static bending were examined at two 10-year-old clonal trials located at Windsor and St-Ours, southern Quebec, Canada. The materials consisted of three hybrids, Populus deltoides × Populus nigra, Populus trichocarpa × P. deltoides, Populus maximowiczii × Populus balsamifera, and native P. deltoides. Significant differences were observed in mechanical properties among hybrids and P. deltoides. The effects of growth on the mechanical properties were inconsistent and varied considerably by site and by hybrid. Results indicated no uniform trends relating growth rate to either higher or lower modulus of elasticity/modulus of rupture. It appears that selection for strength properties may not uniformly lead to decreased growth production, especially for P. trichocarpa × P. deltoides and P. maximowiczii × P. balsamifera.