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Dissemination of antimicrobial resistance in food animals is a One Health concern, but sheep production has been overlooked. This study aimed to explore the dissemination of Escherichia coli resistant to extended-spectrum cephalosporins (ESC) in feedlot lambs. Two pens were sampled on two occasions, and carcasses and other mammals living around were also tested. E. coli were recovered and antibiotic resistance determined. bla CTX-M/CMY genes and their genetic localization were characterized. Whole genome sequencing (WGS) was performed to confirm clonal relationship. The most prevalent ESC-resistance genes in the 108 E. coli isolates were bla CTX-M-55 (53.7 %), bla CTX-M-2 (14.8 %) and bla CMY-2 (13.9 %). Most bla CTX-M-55 genes were found on the chromosome, but IncA/C, IncHI1, IncHI2 and IncF plasmids were also identified. Genetic diversity was observed even though ST6448 was by far the most frequent ST. WGS analysis showed high similarity among isolates recovered from feedlot lambs, animals in the surroundings and lambs' carcasses, proving the clonal and plasmidic dissemination. • 10 %-37 % of lambs intended for human consumption carried ESC-resistant E. coli. • The ST6448-B1 clone was highly prevalent (50/108 isolates). • Plasmidic transfers were suspected for IncHI2-ST2/ bla CTX-M- and IncC-ST3/ bla CMY-2. • Identical isolates were identified in lambs, carcasses and surrounding animals. [ABSTRACT FROM AUTHOR]