The streptomycin resistance of Escherichia coli 2418 strain has been shown to be associated with a 1.2-kb DNA fragment found in the naturally occurring plasmid R2418S. Here, nucleotide sequence analysis of the 1.2-kb DNA fragment revealed the presence of the strB gene which is located immediately downstream of the strA gene. Both sequences are identical to those of strA and strB genes in plasmid RSF1010. Thus, the observed resistance in the clinical isolate is due to the presence of strA-strB genes encoding streptomycin-modifying enzymes. The sequence downstream of strB gene showed a perfect homology with that of RSF1010. In addition, it contained the right inverted repeat of the transposon Tn5393 that has been suggested to be a relic of this transposon found in DNA plasmids isolated from human- and animal-associated bacteria.