Carbapenems have a potent activity against and are often used as last resort for the treatment of infections due to multiresistant Acinetobacter baumannii isolates. The first carbapenem-resistant isolates in Croatia were found in University Hospital Split in 2002 The carbapenem-resistance was mediated by hyperproduction of OXA-51 due the ISAba1 located upstream of the genes. Later, the studies of carbapenem-resistance in Croatia reported OXA-72 β-lactamase in University Hospital Center Zagrebit and University Hospital Split Zagreb. These observations gave rise to a multicenter study conducted in 2009. in Northern Croatia and Istria. The aim of the present study was to characterize the genetic basis of carbapenem resistance and to compare the genotypes of the isolates. In total, 185 isolates were collected from 13 centers in Northern Croatia and Istria. Antibiotic susceptibilities were determined by broth microdilution method according to CLSI. Genes encoding carbapenem hydrolyzing-oxacillinase, metallo-beta-lactamases (MBLs) of VIM, IMP and SIM series and extended-spectrum beta-lactamases (ESBLs) belonging to PER and TEM family were detected by PCR and when needed identified by sequencing. Sequence groups SG corresponing to international clonal lineages (ICL I-III) were determined by multiplex PCR and genotyping of the strains was performed by MLST (multilocus sequence typing), pulsed-field gel electrophoresis (PFGE) and random amplification of polymorphic DNA (RAPD). Thirty-six strains (19.5%) were found to produce acquired oxacillinases. The most prevalent group was OXA-24 with 17 positive isolates. Sequencing of blaOXA-24 gene from the representative strain identified OXA-72 β-lactamase. Fifteen isolates were positive for OXA-58 and four for OXA-23. Sequencing of the blaOXA-58 gene from the representative strain identified OXA-58 β-lactamase. The remaining 148 strains possessed only the naturally ocurring OXA-51 β-lactamase which was associated with ISAbaI insertion sequence upstream of blaOXA-51 gene in 118 of them. They were assigned to SG 2 (ICL I). No MBLs were found. Only OXA-58 β-lactamase was inhibited by sodium chloride. Chromosomal AmpC β-lactamases did not affect the susceptibility to carbapenems. ISAbaIII was found upstream of blaOXA-58 gene in all strains positive for this gene. The blaOXA-51 genes has been sequenced and OXA-69 and OXA-107 β-lactamase were found in the strains producing only OXA-51 β-lactamase and belonging to ICL I, while strains positive for acquried oxacillinases belonging to ICL II were found to possess OXA-66 β-lactamase as intrinsic, chromosomal β-lactamase. Strains possessing acquired oxacillinases were obtained from five large hospitals in Zagreb. PFGE revealed two major clones with more than 85% similarity of the banding patterns: one from University Hospital Center in Zagreb producing OXA-24/40 group of β-lactamase and the other from Pula possessing only chromosomal OXA-51 group of β-lactamase. This study revealed diversity among carbapenemases produced by A. baumannii from different geographic regions in Croatia. Carbapenem-resistant strains positive for acquired oxacillinases originated mostly from large hospital centers. OXA-24/40 was the most prevalent group of acquired oxacillinases. Most of the strains with acquired oxacillinases belonged to ICL II. Outpatients' isolates were found to possess only the naturally occuring OXA-51 β-lactamase.