In order to study the resistance to â-lactam antimicrobials and the mechanisms involved in Salmonella infantis strains in the Northeast of Argentina, twenty nine strains isolated from faeces between February and December 1998 were studied. All strains were resistant to ampicillin, cephalothin, and ampicillin/clavulanic acid but susceptible to cefoxitin. Ten strains were resistant to cefotaxime. A â-lactamase compatible with TEM-1-like were found in all isolates. An additional CTX-M-2-like â-lactamase was detected only in the cefotaxime-resistant strains. Mating experiments confirmed that this resistance genes were located on a conjugative plasmid harbouring one class I integron. Characterisation of ESBL-type by molecular methods could conduce to a better understanding of the evolution and spreading of the â-lactamases family and can alert about the need to adequate the use of antimicrobials in humans as in animal applications.