<p>PPO inhibitors are a valuable group of herbicides that provide soil-residual and foliar control of glyphosate resistant <em>Amaranthus</em> species. The ΔG210 mutation in the <em>PPX2</em> gene confers PPO-inhibitor resistance and has been present in the Midwest for more than a decade. Until recently, PPO-inhibitor resistance in waterhemp was attributable to just the ΔG210 mutation in the <em>PPX2</em> gene, but recently, several new PPO-resistant biotypes have been discovered in waterhemp and Palmer amaranth. A possible explanation is a change in PPO-inhibitor use patterns and commonly used active ingredients.</p> <p>A direct comparison of the ΔG210 mutation and a new mutation, R128G, was conducted in the greenhouse. Results indicate that the R128G mutation in waterhemp is not substantially better than the ΔG210 mutation with respect to conferring resistance to PPO inhibitors applied preemergence. Furthermore, there is no evidence that the utility of PPO inhibitors applied preemergence will diminish any further as a result of the R128G mutation increasing in frequency. A set of field trials was conducted to investigate how a new PPO inhibitor, trifludimoxazin, will select for resistant biotypes in the field. Overall, a greater number of resistant plants survived the foliar herbicide applications than emerged through soil applications. Trifludimoxazin did not increase the frequency of PPO-resistant individuals when applied to soil, but when applied to foliage, increased the frequency of PPO-resistant individuals by 2.5 to 2.6-fold, similar to other PPO inhibitors applied to foliage. In other experiments, investigations of waterhemp populations with unique resistance phenotypes were conducted. In populations that were more resistant than others, yet had the same ΔG210 mutation, there was no evidence of herbicide metabolism contributing to the greater resistance phenotype. In resistant populations that had no target site mutations, target site expression experiments and lipid peroxidation experiments were inconclusive. However, there was some evidence of increased target-site expression or increased antioxidant capacity as being causal mechanisms, although no mechanisms have been fully ruled out.</p>