An unusual class 1 integron was identified that carries an IS26-disrupted aadA1 gene cassette (designated as 'integron-IS26') in an imipenem-resistant Acinetobacter baumannii (IRAB) outbreak strain. DNA sequencing revealed that integron-IS26 contained two gene cassettes, the aac(6')-Im cassette and a peculiar aadA1 cassette that was disrupted by IS26 (disrupted aadA1 cassette). Southern blotting localised integron-IS26 to the chromosome. Nested polymerase chain reaction (PCR) was performed to define the frequency of integron-IS26 in five groups of bacteria. Nested PCR identified integron-IS26 in 19 (73.1%) of 26 clinical outbreak strains of IRAB, 10 (100%) of 10 IRAB isolated from environmental cultures, 3 (13.0%) of 23 imipenem-susceptible A. baumannii (ISAB) non-outbreak strains, 1 (3.6%) of 28 netilmicin- and tobramycin-resistant A. baumannii and none of the netilmicin- and tobramycin-resistant Pseudomonas aeruginosa. In conclusion, we have identified a novel class I integron that carries the aac(6')-Im cassette and an IS26-disrupted copy of aadA1 (integron-IS26) in most IRAB outbreak strains and in a few ISAB non-outbreak control strains. Integron-IS26 is located chromosomally.