Copies of an insertion-sequence (IS)-like element were isolated from two closely related serovars of Leptospira interrogans sensu stricto. Nucleotide sequence analysis of the 1236 bp element showed a characteristic IS structure with terminal imperfect inverted repeats (IRs) flanking a 1159 bp central region. This element was designated IS1500. Four open reading frames (orfA-orfD) were found in the central 'unique' region of IS1500. Similarities were detected between ORFA and ORFB and the putative transposases from members of the IS3 family of transposable elements. IS1500 or IS1500-like sequences were also detected in all other pathogenic leptospiral serovars, but not in the saprophytic species L. biflexa. Differences in IS1500 copy numbers in members of the same species suggest that this element can transpose. Physical mapping of IS1500 insertions in L. interrogans serovars icterohaemorrhagiae and pomona showed insertions were only on the large chromosomal replicon. The location of some IS1500 insertions coincides with regions of the genome that have undergone large rearrangements.