Herbicide resistance may be useful for reducing costs and environmental impacts, and improving yields, during weed control in poplar plantations.  However, genetically engineered traits can sometimes show instability, which would compromise their commercial value.  To study the stability of herbicide resistance, we analyzed resistance to the contact herbicide glufosinate in 384 transgenic plants originating from 32 gene-insertion events created within two Populus hybrids (P. tremula × P. alba and P. tremula × P. tremuloides).  Resistance was measured at the start and end of an 8-year period during which trees were cut and allowed to resprout in the field several times.  The transgenic events had widely varying levels of resistance, ranging from complete tolerance to nearly complete sensitivity.  When analyzed as three classes (tolerant, intermediate, and sensitive), the levels of resistance remained stable, and there were no cases of resistance breakdown.  The level of the resistance-conferring PAT protein, based on enzyme-linked immunosorbent assays, was strongly correlated with resistance class; thus, simple protein assays should enable early screening for acceptable resistance levels.  Our data suggest that commercial levels of herbicide resistance and stability can be introduced into elite clones of hybrid poplar with as little as 2-3 years of transformation and field testing.