A clinical strain of Acinetobacter calcoaceticus resistant to carbapenems was isolated from a blood culture sample from an inpatient in a hospital in Madrid (Spain) during a large outbreak of infection (affecting more than 300 inpatients), caused by a multidrug-resistant Acinetobacter baumannii clone. The carbapenem resistance in both the A. calcoaceticus and A. baumannii clones was due to a blaOXA-24 gene harbored in different plasmids. The plasmids were fully sequenced, revealing the presence of site-specific recombination binding sites putatively involved in mobilization of the blaOXA-24 gene. Comparison of plasmids contained in the two strains revealed possible horizontal transmission of resistance genes between the Acinetobacter species. Copyright © 2010, American Society for Microbiology. All Rights Reserved.