Aims To determine the herd prevalence of Enterobacteriaceae producing CTX-M-type extended-spectrum -lactamases (ESBLs) among 381 dairy farms in Japan. Methods and Results Between 2007 and 2009, we screened 897 faecal samples using BTB lactose agar plates containing cefotaxime (2gml-1). Positive isolates were tested using ESBL confirmatory tests, PCR and sequencing for CTX-M, AmpC, TEM and SHV. The incidence of Enterobacteriaceae producing CTX-M-15 (n=7), CTX-M-2 (n=12), CTX-M-14 (n=3), CMY-2 (n=2) or CTX-M-15/2/14 and CMY-2 (n=4) in bovine faeces was 28/897 (3 center dot 1%) faecal samples. These genes had spread to Escherichia coli (n=23) and three genera of Enterobacteriaceae (n=5). Herd prevalence was found to be 20/381 (5 center dot 2%) dairy farms. The 23 E.coli isolates showed clonal diversity, as assessed by multilocus sequence typing and pulsed-field gel electrophoresis. The pandemic E.coli strain ST131 producing CTX-M-15 or CTX-M-27 was not detected. Conclusions Three clusters of CTX-M (CTX-M-15, CTX-M-2, CTX-M-14) had spread among Japanese dairy farms. Significance and Impact of the Study This is the first report on the prevalence of multidrug-resistant CTX-M-15-producing E.coli among Japanese dairy farms.