Salmonella ordonez (BM 2000) codes for kanamycin (Km, aphA), ampicillin (Ap), streptomycin (SmSp:aadA and Sm:aphC), chloramphenicol (Cm), tetracycline (Tc) and sulfonamide (Su) resistances and for production of colicin Ib (Cib). Genetical analysis by incompatibility testing, conjugation, transformation and physical studies using electron microscopy, agarose gel electrophoresis, led us to associate the Km and Cib characters to a 98.7 kilobase (kb) IncI1 plasmid (pIP565), and the Sm (aphC) and Su determinants to a 8.3 kb plasmid (pIP605). The ApCmSmSp(aadA)SuTc determinants were not associated in BM2000 S. ordonez with a plasmid structure. Following conjugation of S. ordonez to E. coli, the ApCmSmSpSuTc determinants were found stably associated with a single plasmid structure (pIP173, 127.5 kb) belonging to IncI1 group. Agarose gel electrophoresis of plasmid DNA restriction endonuclease digests and electron microscopy heteroduplex analysis showed that the acquisition of the ApCmSmSpSuTc determinants resulted from the insertion into pIP565 of a 28.8 kb DNA sequence. This sequence coding for ApCmSmSpSuTu resistances in S. ordonez could be translocated either to pIP565 plasmid or to several IncI1 plasmids but never to plasmids belonging to IncW, IncP or IncFII, suggesting the existence of specific sequences on the IncI1 receptor plasmids. Moreover, R-determinants were translocated back "en bloc" from pIP173 to the chromosome of a susceptible S. ordanez. The results were consistent with the presence in BM2000 S. ordonez chromosomal DNA of an integrated translocatable sequence encoding ApCmSmSpSuTc resistances. Such a structural association could account for the stability of these resistances in the Salmonella ordonez serotype.