We have discovered transposase sequences in the bull frog (Rana catesbeiana) and in the clawed frog (Xenopus laevis), which demonstrates that there are DNA-mediated transposons inAmphibia. The DNA sequences of 11 newXenopuselements describe two new vertebrate transposon families. Phylogenetic analysis, using these sequences along with pre- viously defined vertebrate and invertebrate elements, reveals at least five families of Tc1-like elements inVertebrata. Some of these families co-exist in the same genome. Furthermore, the grouping of one of the amphibian transposon families with a branch of the teleost transposons raises the possibility of horizontal transfer.