<jats:p>Two pairs of ceftolozane/tazobactam susceptible/resistant <jats:italic>P. aeruginosa</jats:italic> were isolated from 2 patients after exposure to <jats:italic>β</jats:italic>-lactams. The genetic basis of ceftolozane/tazobactam resistance was evaluated, and <jats:italic>β</jats:italic>-lactam-resistant mechanisms were assessed by phenotypic assays. Whole genome sequencing identified mutations in AmpC including the mutation (V213A) and a deletion of 7 amino acids (P210–G216) in the Ω-loop. Phenotypic assays showed that ceftolozane/tazobactam resistance in the strain with AmpC<jats:sub>V213A</jats:sub> variant was associated with increased <jats:italic>β</jats:italic>-lactamase hydrolysis activity. On the other hand, the deletion of 7 amino acids in the Ω-loop of AmpC did not display enhanced <jats:italic>β</jats:italic>-lactamase activity. Resistance to ceftolozane/tazobactam in <jats:italic>P. aeruginosa</jats:italic> is associated with changes in AmpC; however, the apparent loss of <jats:italic>β</jats:italic>-lactamase activity in AmpC∆7 suggests that non-AmpC mechanisms could play an important role in resistance to <jats:italic>β</jats:italic>-lactam/<jats:italic>β</jats:italic>-lactamase inhibitor combinations.</jats:p>
