Extended-spectrum β-lactamase (ESBL)-mediated resistance is of considerable importance in human medicine. Recently, such enzymes have been reported in bacteria from animals. We describe a longitudinal study of a dairy farm suffering calf scour with high mortality rates. In November 2004, two Escherichia coli isolates with resistance to a wide range of β-lactams (including amoxicillin-clavulanate and cefotaxime) were isolated from scouring calves. Testing by PCR and sequence analysis confirmed the isolates as being both of blaCTX-M14/17 and blaTEM-35 (IRT-4) positive. They had indistinguishable plasmid and pulsed-field gel electrophoresis (PFGE) profiles. Transferability studies demonstrated that blaCTX-M was located on a conjugative 65-MDa IncK plasmid. Following a farm visit in December 2004, 31/48 calves and 2/60 cows were positive for E. coli with bla CTX-M. Also, 5/48 calf and 28/60 cow samples yielded bla CTX- and blaTEM-negative E. coli isolates that were resistant to cefotaxime, and sequence analysis confirmed that these presented mutations in the promoter region of the chromosomal ampC gene. Fingerprinting showed 11 different PFGE types (seven in blaCTX-M· positive isolates). Six different PFGE clones conjugated the same blaCTX-M- positive IncK plasmid. One clone carried a different-sized, of bla CTX-M·positive, transformable plasmid. This is the first report of blaCTX·M from livestock in the United Kingdom, and this report demonstrates the complexity of ESBL epidemiology. Results indicate that horizontal plasmid transfer between strains as well as horizontal gene transfer between plasmids have contributed to the spread of resistance. We have also shown that some clones can persist for months, suggesting that clonal spread also contributes to the perpetuation of resistance.