Objectives Linked blaCTX-M-3–blaTEM-1b genes, as found on the IncI1 plasmid pEK204 prevalent in Belfast, also occur on plasmids belonging to other rep types in Escherichia coli isolated from nursing-home residents in Belfast. We investigated the mechanisms for their joint dissemination among diverse plasmids. Methods Plasmid pEK204 was transferred by electroporation into E. coli DH5a harbouring derivative pBAD Myc-His vectors. Transposition experiments were then performed at 37°C. Transposition of bla genes onto the derivative pBAD Myc-His vector was confirmed by sequencing. Results ISEcp1 mediated transposition of blaCTX-M-3 alone from pEK204, as well as both blaCTX-M-3 and blaTEM-1b jointly. The 5' and 3' termini of the transposed fragments were identical to or resembled the ISEcp1 IRL and IRR, respectively, thereby replicating the environments previously found in various Belfast clinical plasmids harbouring blaCTX-M-3. Conclusions Simultaneous dissemination of blaCTX-M-3 and blaTEM-1b among plasmids in Belfast's nursing homes is facilitated by ISEcp1-mediated transposition of these bla genes from pEK204-like and other plasmids. Such transposition events are of public health concern, as they potentially allow wider dissemination of CTX-M-3 enzyme than would be possible through the spread of extended-spectrum ß-lactamase-encoding plasmids and strains alone.