The tdh (thermostable direct hemolysin) gene occurs in some strains of Vibrio species. All tdh genes are flanked by insertion sequence-like elements (ISV). All previous attempts have failed to detect transposition of these ISVs. In this work, we have built a transposition detection system in E. coli and succeeded in detecting the transposition of an insertion sequence-like element at a frequency of Km(r) mutants of 7.2 x 10(-6). A specific flanking sequence (5'-Py-Pu-3') was found on either side of the target duplication.