The tetracycline resistance gene (tet) from the Campylobacter jejuni plasmid pFKT1025 was cloned into both pUC18 and pBR322 and was expressed when the chimeric plasmids were introduced into Escherichia coli. The location of the tet determinant on the chimeric plasmids was determined by BAL 31 deletion mapping within a 2.25-kilobase (kb) RsaI-HincII fragment. A protein of approximately 70 kilodaltons was consistently produced by E. coli maxicells harboring the cloned tet determinant. A 500-base-pair restriction fragment from within the 2.25-kb tet region was shown to hybridize only to DNA from tetracycline-resistant strains of C. jejuni and C. coli, but not to the DNA of organisms known to carry the streptococcal tetM determinant. No homology was noted between the DNA of 10 tetracycline-resistant isolates of campylobacter and the streptococcal tetL, tetM, or tetN determinants when tested under conditions of high stringency. However, homology was noted between a 5.0-kb HincII restriction fragment containing the tetM determinant and two C. jejuni tet R factors under conditions of reduced stringency.