Campylobacter coli strain BM2509 resistant to ampicillin, chloramphenicol, erythromycin, kanamycin, spectinomycin, streptomycin and tetracycline was isolated from a patient with hospital-acquired diarrhoea. Resistance to kanamycin has not been thus far described in Campylobacter. Phosphocellulose paper-binding assay indicated that resistance to kanamycin and structurally related antibiotics in strain BM2509 was due to synthesis of a 3'-aminoglycoside phosphotransferase of type III (APH(3')-III), an enzyme so far confined to Gram-positive cocci. The kanamycin and tetracycline resistances were transferable en bloc by conjugation to C. fetus but not to Escherichia coli. Analysis by agarose gel electrophoresis of crude bacterial lysates revealed the presence, in BM2509 and in the transconjugants, of a plasmid, pIP1433, with a size of 47.2 kilobases (Kb). Strain BM2509 also harboured a 4.5-Kb cryptic plasmid. DNA annealing studies indicated a close structural relationship between the kanamycin resistance gene of C. coli BM2509 and that representative of this type of resistance determinant in Gram-positive cocci. These results indicate that emergence of resistance to kanamycin in Campylobacter is due to acquisition in vivo of a gene or a plasmid from Gram-positive bacteria.