Four sequence type 664 (ST664) (serotype O:5) strains of Pseudomonas aeruginosa that were highly resistant to antibiotics, including ceftolozane-tazobactam and ceftazidime-avibactam, but were susceptible to colistin were found to harbor the gene encoding the rare class C β-lactamase PAC-1 on a chromosomally located Tn 1721 -like transposon. The bla PAC-1 gene was associated with the 16S rRNA methylase determinant rmtF2 , which confers pan-aminoglycoside resistance.