Plasmids conferring mupirocin resistance were prepared from isolates of Staphylococcus aureus obtained from four patients in the same ward. The plasmids are related and in all of them the gene conferring mupirocin resistance (mupA) is flanked by copies of IS257 in direct repeat. In two plasmids mupA and IS257 have been duplicated and in one of these plasmids (pJ3358) a small pT181-like plasmid conferring tetracycline resistance is present flanked by copies of IS257. Filter mating with a strain containing pJ3358 as donor and selection on tetracycline sometimes resulted in transfer of the pT181-like plasmid containing a copy of IS257. Analysis showed that the pT181-like plasmid with the insertion of IS257 is present in high copy number and that the IS257 element is inserted in the copy number control region of the plasmid.