Insertion sequence IS100 was localized on a 9.5-kb plasmid of Yersinia pestis and was shown to be specific for Y. pestis and serotype I strains of Y. pseudotuberculosis. The nucleotide sequence of IS100 isolated from this plasmid was determined. The element, which was flanked by 5-bp direct repeats, contained 1953 bp including imperfect inverted terminal repeats of 52 and 61 bp long (43 bp were identical). Two open reading frames encoding potential polypeptides of 340 and 252 amino acids were identified on one DNA strand. Nucleotide sequence as well as deduced polypeptide sequences of IS100 were homologous to those of IS21, IS232 and IS640.