The positions of the outer boundaries of the 5'- and 3'-conserved segment sequences of integrons found at several different locations have been determined. The position of the 5' end of the 5'-conserved segment is the same for six independently located integrons, In1 (R46), In2 (Tn21), In3 (R388), In4 (Tn1696), In5 (pSCH884), and In0 (pVS1). However, the extent of the 3'-conserved segment differs in each integron. The sequences of In2 and In0 diverge first from the conserved sequence, and their divergence point corresponds to the 3'-conserved segment endpoint defined previously (H.W. Stokes and R.M. Hall, Mol. Microbiol. 3:1669-1683, 1989), which now represents the endpoint of a 359-base deletion in In0 and In2. The sequence identity in In3, In1, In4, and In5 extends beyond this point, but each sequence diverges from the conserved sequence at a different point within a short region. Insertions of IS6100 were identified adjacent to the end of the conserved region in In1 and 123 bases beyond the divergence point of In4. These 123 bases are identical to the sequence found at the mer end of the 11.2-kb insertion in Tn21 but are inverted. In5 and In0 are bounded by the same 25-base inverted repeat that bounds the 11.2-kb insert in Tn21, and this insert now corresponds to In2. However, while In0, In2, and In5 have features characteristic of transposable elements, differences in the structures of these three integrons and the absence of evidence of mobility currently preclude the identification of all of the sequences associated with a functional transposon of this type.