Objectives We investigated the molecular epidemiology of carbapenem-resistant Acinetobacter baumannii from a Taiwanese hospital and determined the mechanisms responsible for resistance. Methods Ninety-two consecutive meropenem-resistant A. baumannii isolates collected between January 2005 and June 2007 were screened for genes encoding OXA carbapenemases, metallo-ß-lactamases and for the carO gene encoding an outer membrane protein. PFGE was used to define clonal relatedness. PCR mapping was used to examine the linkage of insertion sequences and blaOXA genes. Southern hybridization of plasmid extracts and I-CeuI-restricted chromosomal DNA was used to locate blaOXA-24-like genes. Sequences of selected blaOXA-24-like and carO genes were determined and loss of CarO expression was confirmed by SDS–PAGE. Results Most (70/92, 76%) isolates belonged to one of three PFGE pulsotypes, indicating clonal spread. Fifty-nine isolates, including the majority of those of pulsotypes I and III, produced OXA-72 carbapenemase. The blaOXA-72 gene was located on a 54 kb plasmid in selected isolates. Thirty-three (36%) isolates, including all 16 of pulsotype II, had ISAba1 preceding the blaOXA-51-like gene, promoting its expression. In addition to OXA-72 carbapenemase, two pulsotype I and three pulsotype III isolates did not produce CarO protein as the carO gene was disrupted by insertion of an ISAba1 element. Two isolates of a minor pulsotype had a blaOXA-58-like gene and a single PFGE-unique isolate had a blaOXA-23-like gene. Conclusions Although diverse mechanisms were identified, production of OXA-72 carbapenemase was the most common mechanism of carbapenem resistance in A. baumannii from this Taiwanese hospital. The plasmidic location of the gene had facilitated its spread to multiple strains.