Forty-two strains of Acinetobacter baumannii were isolated from 15 patients hospitalized in a French intensive care unit. An epidemiological study based on the typing of these isolates was carried out using biotyping, antibiotyping, and ribotyping to recognize the transmission of multiresistant strains by transfer of a patient from one hospital to another. Fifteen strains from the outbreak (1 strain for each patient), five Acinetobacter baumannii strains isolated before the outbreak in Bellevue Hospital (St. Etienne), and five strains isolated in Cochin Hospital (Paris) were included. The three methods gave a good correlation: the epidemic strains had the same antibiotic resistance pattern, the same biotype, and the same ribotypes obtained with three different endonucleases.