Five clinical isolates of Gardnerella vaginalis known to carry the tetracycline resistance determinant Tet M were examined by hybridization and nucleotide sequencing. Four of the strains carried tet(M) genes with identical sequences. The two versions of the tet(M) gene found in G. vaginalis did not show complete identity with other published tet(M) sequences, but showed mosaic structures with regions of homology to tet(M) gene sequences from Tn916, Tn1545 and the American type plasmid found in Neisseria gonorrhoeae. Hybridization studies showed that all isolates carried the tet(M) gene on a single HindII restriction fragment of variable length. No evidence was found for the presence of sequences homologous to the transposition functions of Tn916.