During a period of 11 months Acinetobacter baumanii was isolated from 27 patients and 21 environmental samples in the Intensive Care Unit (ICU) and in one surgical unit of a University Hospital. The isolates were characterized by biotyping, antibiograms and plasmid profiles and compared with co-isolates. Plasmid fingerprinting distinguished three outbreaks, whereas other typing methods were less sensitive and discriminatory. Although plasmid profiles seem to be a simple and reproducible marker for epidemiological studies with acinetobacter strains, it might be useful to combine at least two typing methods since plasmids are unstable genetic structures, and not all strains possess plasmids.