A new insertion element present in two alleles, designated IS1635.1 and IS1635.2, was identified on a plasmid of a Yersinia intermedia strain by hybridization with the Yersinia enterocolitica pYV virulence plasmid. IS1635.1 and IS1635.2 are 861 bp long, carry imperfect inverted terminal repeats and possess a single open reading frame encoding a putative transposase of the IS6 family. A truncated IS1635 element is present immediately downstream of element IS1635.2. The capacity of the IS1635 elements to mediate transposition in Yersinia was demonstrated with a R6K-derived suicide vector, where a kanamycin resistance gene had been inserted between IS1635.1 and IS1635.2. Hybridization and sequence alignments showed that remnants of IS1635-like insertion elements harboring large deletions and point mutations are present on the Yop virulon harboring plasmids of pathogenic Yersinia strains. In a few cases, the IS1635 element has also been found on plasmids of apathogenic Yersinia strains. ß