Résumé : Virulence-associated plasmids pLT2, pIP1350, pIP1360 and pIP1370 coded for the fi+ character. These plasmids did not code for the synthesis of classical adhesins, aerobactin or colicins. Nor did they mediate resistance to the metals Hg, Te, As, Cd or flavomycin. Plasmids pLT2, pIP1350 and pIP1370 contained DNA sequences homologous to the incFIIA and incFIB replicons. Plasmid pIP1360 contained a single sequence homologous to incFIIA replicon.