Ten aminoglycoside-resistant Escherichia coli isolated from the faeces of healthy or diarrhoeic animals reared in the same herd were studied. These strains were resistant to high levels of apramycin and low levels of gentamicin. They were also resistant to streptomycin, tetracycline, trimethoprim and some to ampicillin, chloramphenicol, kanamycin or nalidixic acid. Two strains, isolated from a calf and a lamb, respectively, belonged to the same biotype. All the transconjugants resistant to gentamicin-apramycin were also resistant to streptomycin, tetracycline and trimethoprim. In all cases, these resistances were encoded by plasmids of 100 kb. Analysis of these plasmids by agarose gel electrophoresis after digestion by EcoRI or BamHI revealed their similarity. Hybridization with a 500-bp HpaI insert of plasmid pFE872 was observed with DNA from field strains and their transconjugants, demonstrating the presence on the 100-kb plasmids of the gene coding for a dihydrofolate reductase I. A single plasmid, designated pIP1831, could be observed in identical or different strains isolated from calves or lambs, suggesting the transmission of strains and plasmids between animals of different species.