The class I integrons In0, In2, and In5, found in different locations in pVS1, Tn21, and pSCH884, have closely related structures. All three integrons contain an insertion sequence, IS1326, that is a new member of the IS21 family. IS1326 has caused deletions of adjacent 3'-conserved segment and transposition module sequences, and all three integrons retain a complete copy of only one of four genes required for transposition of related transposons and are thus defective transposon derivatives. In2 contains an additional insertion sequence, IS1353, located within IS1326. IS1353 is a member of the IS3 family and appears to have been acquired after the integron was inserted into an ancestral mercury resistance transposon to create the ancestor of Tn21 and several other transposons that are close relatives of Tn21.