Of 23 ceftazidime-resistant Pseudomonas aeruginosa isolates with simultaneous decreased susceptibility to carbapenems recovered from inpatients at Hospital Eva Peron, Provincia de Buenos Aires, Argentina, from December 2004 to December 2005, 18 were positive by a double-disk phenotypic screening of metallo-β-lactamases (MBLs) using EDTA (1 mol). Decreased susceptibility was defined as isolates with MICs of 2 to 8 μg/ml or inhibition zones of 16 to 21 mm, categorized as susceptible or intermediate according to CLSI breakpoints but differing from fully susceptible isolates. All of these isolates were susceptible to piperacillin, piperacillin-tazobactam, and colistin; 13 of 18 were susceptible to amikacin, while 5/18 were intermediate. They were all resistant to ceftazidime, cefepime, gentamicin, and ciprofloxacin. Fourteen displayed decreased susceptibility to imipenem (IPM) and meropenem (MEM) (inhibition zone range, 16 to 21 mm), while 4 consecutive isolates were fully resistant to both. Independently of their categorization, pulsed-field gel electrophoresis (PFGE) profiles using 20 U/plug SpeI nuclease were identical after standard resolution. These 18 isolates were positive when screened for the class 1 integrase gene and rendered a unique fragment corresponding to the variable region of class 1 integrons, in which imp-13 was the first cassette, followed downstream by an aminoglycoside-modifying enzyme-coding gene, aacA4 (99% identical to that already deposited for Tn5051). Because a difference in final carbapenem resistance could be due to different mechanisms, including differential β-lactamase content and expression, crude extracts were examined after analytical isoelectrofocusing to detect enzymatic activity.