In Klebsiella pneumoniae, it is common for plasmid-located and chromosome-located bla(SHV) copies to coexist within single cells. The plasmid-borne genes are mainly derived from two separate IS26-mediated mobilizations of bla(SHV). The objective of this study was to test the hypothesis that the presence of a non-extended-spectrum beta-lactamase (non-ESBL) encoding plasmid-borne form of bla(SHV) facilitates the cefotaxime (CTX)-mediated selection of ESBL-expressing mutants, even when there is a chromosomal copy of the same gene. Twenty-one diverse ESBL-negative, bla(TEM)-negative K. pneumoniae clinical isolates were tested for the IS26 insertions characteristic of the two mobilization events. The isolates were then tested for their ability to be selected for ESBL-mediated CTX resistance by serial subculturing with a doubling of the CTX concentration at every subculture. Fourteen isolates possessed neither of the IS26 insertions. None of these became ESBL positive, and all died during the course of the experiment, despite possessing chromosomal bla(SHV) copies. The other isolates all became ESBL positive and grew abundantly up to a CTX concentration of 128 microg/ml. Similar results were obtained with ceftazidime. ESBL expression was associated with the appearance of the expected G-->A mutation at position 1 of codon 238 and also with bla(SHV) copy number amplification. It was concluded that plasmid-borne bla(SHV) greatly facilitates the selection of ESBL expression, even when the same gene is on the chromosome, and that gene dosage effects are likely to contribute to this phenomenon.