Escherichia coli2418 strain is resistant to β-lactam antibiotics (ampicillin, carbenicillin, and cephalothin), streptomycin, tetracycline, kanamycin, and chloramphenicol. This strain contains at least two conjugative plasmids (R2418 and R2418S) encoding resistance to β-lactam antibiotics and resistance to both β-lactam antibiotics and streptomycin, respectively. Restriction endonuclease mapping of plasmid DNAs indicates that the plasmid R2418S has evolved from R2418 DNA by the insertion of 2.5-kb DNA betweenBamHI andPvuII sites, and deletion of 0.5-kb DNA within theEcoRI–EcoRV region. The 2.5-kb DNA insert is responsible for streptomycin resistance. This evolution is also associated with a reduction in the efficiency of conjugal transfer for the plasmid R2418S. The conjugal transfer of streptomycin resistance occurs only through the coresidence of the conjugative plasmid R2418 or R2418S in the donor cell. In accordance with the hypothesis that the Smrdeterminant is due to a putative transposon, plasmid-free transconjugants resistant to streptomycin only were isolated. Southern blot analysis ofHindIII chromosomal digests extracted from these transconjugants shows that the Smrdeterminant is inserted into different sites in chromosomal DNA.Key words:Escherichia coli, antibiotic resistance, conjugation, transformation, plasmid, transposon, restriction map.