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1. Bacterial resistance to β-lactam antibiotics has risen dramatically inEscherichia colifrom food animals. In a previous study, 29 randomly selected chicken products, collected in Portugal, were analysed for the presence of extended-spectrum β-lactamases (ESBLs)-producingE. coli; and during this study the genetic characterisation of ESBLs genes was investigated. 2. The presence of genes encoding TEM, OXA, SHV, and CTX-M type beta-lactamases was studied by PCR followed by sequencing. Additionally, other mechanisms of antimicrobial resistance, phylogenetic groups and the presence of virulence determinants were evaluated among the isolates. 3. β-lactamases genes were identified as follows:blaCTX-M-14(n = 4),blaCTX-M-1(n = 2),blaCTX-M-9(n = 4) andblaTEM-52(n = 13). Mutations at positions −42, −18, −1, and +58 ofampCpromoter region were identified in 4 non-ESBL-producing isolates. Thetet(A) ortet(B) genes were identified in all tetracycline-resistant isolates; theaadAgene detected in 8 of 10 streptomycin-resistant isolates; theaac(3)-II gene in all gentamicin-resistant isolates; thecmlAgene in the chloramphenicol-resistant isolate; andsul1and/orsul2 and/orsul3genes were found in all trimethoprim-sulfamethoxazole-resistant isolates. TheintI1gene was detected in 8 trimethoprim-sulfamethoxazole-resistant isolates and theintI2gene in 4 isolates; one gene cassette arrangements were identified among class 1 integrons (dfrA1 + aadA1) and among the class 2 integrons (dfrA1 + sat2 + aadA1). Among cefotaxime-resistant isolates, 16 belonged to A or B1 phylogenetic groups, while 11 isolates were classified into the D or B2 phylogroups. At least one virulence-associated gene (aer,fimA, orpapC) was detected in 74·1% of the cefotaxime-resistant isolates. 4. Because ESBLs-producing bacteria are resistant to a broad range of β-lactams, infections caused by these organisms complicate therapy and limit treatment options. [ABSTRACT FROM AUTHOR]