Two small plasmids of 11.5 and 9.5 kb, each carrying an aphA-7 kanamycin phosphotransferase gene, were studied. The MICs of kanamycin for the two human Campylobacter jejuni isolates harboring the plasmids were 10,000 and 5,000 micrograms/ml, while the MICs of amikacin were 32 and 8 micrograms/ml, respectively. The MICs of gentamicin and tobramycin were less than or equal to 2 micrograms/ml for both isolates. The restriction endonuclease maps of the plasmids were similar, with the larger plasmid showing two discrete regions of additional DNA. When the aphA-7 gene from each plasmid was cloned into pBR322, the aphA-7 gene expressed the kanamycin resistance phenotype in Escherichia coli. For transformants containing the cloned aphA-7 gene, kanamycin MICs were greater than or equal to 128 micrograms/ml. The aphA-7 gene was also subcloned from the plasmid pFKT4420 into the E. coli-Streptococcus shuttle vector pDL278 and was transformed into Streptococcus gordonii Challis. For streptococcal transformants containing the novel plasmid, kanamycin MICs were 4,000 micrograms/ml. In the presence of a tetracycline resistance plasmid, both small plasmids could be mobilized during conjugal matings to Campylobacter coli recipients.