Background and Purpose To investigate the clinical and molecular epidemiology of the imipenem-resistant Acinetobacter calcoaceticus–Acinetobacter baumannii (IRAcb) complex during an outbreak in an intensive care unit (ICU). Methods Forty-six clinical and 11 environmental isolates of the IRAcb complex were collected from the ICU of Taipei Veterans General Hospital, Taiwan between December 2003 and March 2004. These isolates were genotyped using pulsed-field gel electrophoresis (PFGE). Carbapenemase genes and their associated genetic structures were analyzed using PCR. Clinical data obtained from the patients were also reviewed and analyzed. Results The isolates were identified at the genomic species level as A. baumannii (42 clinical and five environmental isolates) and Acinetobacter genomic species 13TU (four clinical and six environmental isolates). Both species were comprised of two pulsotypes, but those of A. baumannii were closely related (83% similar). IS1008-ΔISAba3-blaOXA-58-like and ISAba1-blaOXA-51-like were identified in 22 and 21 clinical isolates of A. baumannii, respectively (one isolate contained both). The ISAba3-bracketed blaOXA-58-like gene was detected in all isolates of Acinetobacter genomic species 13TU. Patient transfers between different sections of the ICU were important factors that contributed to the spread of the two pulsotypes of A. baumannii. However, among the A. baumannii isolates identified, only those carrying IS1008-ΔISAba3-blaOXA-58-like could be found in the environment, indicating an additional route of transmission. The prior use of carbapenem or cefepime was associated with the subsequent infection with A. baumannii carrying the ISAba1-blaOXA-51-like gene, while prior piperacillin/tazobactam use was associated with the subsequent infection with A. baumannii carrying the IS1008-ΔISAba3-blaOXA-58-like gene. Conclusion A. baumannii isolates carrying different carbapenemase genes and their associated genetic structures might be transmitted or selected in different ways.