Escherichia coli strain TUM2139 was isolated from a stool sample from a 9-year-old girl on 16 June 2004. This strain was categorized as Shiga toxin-producing Escherichia coli (STEC) because the Shiga-like toxin gene stx(1) was detected by immunochromatography and PCR assay. The strain was highly resistant to cefotaxime (256 microg/ml) and was also resistant to cefepime, cefpodoxime, ceftriaxone, and aztreonam. In the presence of 4 microg of clavulanic acid per ml, the MIC of cefotaxime decreased to < or =0.12 microg/ml, indicating that this strain was an extended-spectrum beta-lactamase (ESBL) producer. Cefotaxime resistance was transferred to E. coli C600 by conjugation at a frequency of 3.0 x 10(-6). A PCR assay was performed with primer sets specific for TEM-type and SHV-type ESBLs and for the CTX-M-2 (Toho-1), CTX-M-3, and CTX-M-9 groups of ESBLs. A specific signal was observed with the primer set specific for the CTX-M-9 group of beta-lactamases. This beta-lactamase was confirmed to be the ESBL CTX-M-18 by DNA sequencing. This is the first report of an ESBL-producing STEC isolate.