During the COMParative Activity of Carbapenems Testing (COMPACT) surveillance study, 448 Pseudomonas aeruginosa clinical isolates were obtained from 16 Spanish hospitals. Nonsusceptibility (EUCAST breakpoints) to imipenem (35%), meropenem (33%), and/or doripenem (33%) was observed with 175 isolates (39%). Simultaneous resistance to these three drugs was observed with 126 of the 175 isolates (72%). Except for colistin, high resistance rates were observed among noncarbapenem antibiotics. Clonal relatedness was investigated by pulsed-field gel electrophoresis (PFGE) with SpeI, discriminating 68 patterns. Multilocus sequence typing (MLST) was performed on 84 isolates representing different PFGE types and all participating hospitals. Thirty-nine sequence types (STs) could be distinguished, and of these, ST175 (48 isolates, 10 hospitals), ST646 (16 isolates, 4 hospitals), ST532 (13 isolates, 3 hospitals), and ST111 (13 isolates, 7 hospitals) were the most frequently encountered. Minimum-spanning tree analysis confirmed a wide dissemination of different clones among participant hospitals, particularly ST175. PFGE pattern comparison within the four most frequent STs revealed that ST175 isolates were relatively uniform, while ST646, ST532, and ST111 isolates were highly diverse, with almost every isolate belonging to a unique pulsotype, even when originating from the same center. The population of carbapenem-nonsusceptible P. aeruginosa isolates from 16 hospitals is highly diverse, with one ST (ST175) representing a highly conserved clone disseminated in 10 of the 16 participant hospitals. This ST175 clone should be added to the list of P. aeruginosa clones at high risk for epidemic spread, such as the Liverpool, Manchester, and Melbourne clones previously found in cystic fibrosis patients and ST235 in the nosocomial setting.Antibiotic surveillance studies are necessary not only for the better understanding of bacterial epidemiology, but also for the design of control strategies for preventing bacterial resistance and establishing therapeutic guidelines. In this context, the COMParative Activity of Carbapenems Testing (COMPACT) multicenter study was performed in 16 European countries, including Spain (23). The main objectives of this surveillance study were to test the activity of currently used carbapenems against Pseudomonas aeruginosa clinical isolates and to detect carbapenem nonsusceptible isolates (12).In this multicenter study, Spanish authors concluded that percentages of carbapenem resistance among P. aeruginosa clinical isolates in their country (range of 21.4% to 32.6%) are very similar to those previously reported in other European countries. Doripenem was the most active drug against these resistant isolates (12). The presence of carbapenem-nonsusceptible P. aeruginosa isolates has been described worldwide, particularly in relation to outbreaks (1,5,6,17,18,25,28,30,31,37). Most of these studies focused on clinical isolates, and genetic relatedness was established using molecular typing tools based on macror...