Weeds resistant to PPO-inhibiting herbicides threaten the profitability of crop producers relying on this chemistry. InAmaranthus palmeri, mutations at G210 (∆G210) and R128 (R128G/M) of thePPX2gene were reported to confer PPO-inhibitor resistance. Here,A. palmerisamples from nine states in America, having survived a field application of a PPO-inhibitor, were genotyped to determine the prevalence of these mutations. Less than 5% of the 1828A. palmeriplants screened contained the ∆G210 mutation. Of the plants lacking ∆G210, a R128 substitution was only found in a single plant. AnA. palmeripopulation from Alabama without mutations at G210 or R128 had a resistance ratio of 3.1 to 3.5 for fomesafen. Of the candidatePPX2mutations identified in this population, only V361A conferred resistance to lactofen and fomesafen in a transformed bacterial strain. This is the first report of the V361A substitution of PPX2 conferred PPO-inhibiting herbicide resistance in any plant species. Future molecular screens of PPO-inhibitor resistance inA. palmeriand other species should encompass the V361A mutation ofPPX2to avoid false-negative results.