Vaborbactam (formerly RPX7009) is a new β-lactamase inhibitor based on a cyclic boronic acid pharmacophore with potent inhibitory activity against arbapenemases (KPC). It has been developed in combination with meropenem. The objective of these studies was to identify the concentrations of both agents associated with the selection or prevention of single-step mutations leading to reduced sensitivity to the combination and to characterize the selected mutations. Eighteen strains of KPC-producing with various degrees of sensitivity to meropenem (MICs, 8 to 512 μg/ml) and meropenem-vaborbactam (MICs, ≤0.06 to 32 μg/ml) and preexisting resistance mechanisms were selected from a worldwide collection of isolates recovered from surveillance studies, emphasizing strains for which MICs were in the upper range of the meropenem-vaborbactam MIC distribution. Meropenem and vaborbactam at 8 μg/ml each suppressed the drug resistance mutation frequency to <1 × 10 in 77.8% (14/18) of strains, and all strains were inhibited when the meropenem concentration was increased to 16 μg/ml. Mutants selected at lower drug concentrations showed phenotypes associated with previously described carbapenem resistance mechanisms, including inactivation in mutants selected from OmpK36-proficient strains and an increased gene copy number in strains with partially functional No mutations in the coding region of were identified. These data indicate that the selection of mutants with reduced sensitivity to meropenem-vaborbactam from KPC-producing strains is associated with previously described mechanisms involving porin mutations and the increase in the gene copy number and not changes in the KPC enzyme and can be prevented by the drug concentrations achieved with optimal dosing of the combination.