L1 (LINE-1) elements, previously described in several placental species, were shown to exist in several thousands of copies dispersed in the genome of a marsupial (the North American opossum, Didelphis virginiana). A restriction fragment containing the partial nucleotide sequence of the reverse transcriptase gene (ORF-2) from the marsupial L1 element was cloned and sequenced. The element was shown to evolve in concert within the marsupial when compared to placental mammals. When the marsupial L1 element is used as an outgroup to root a phylogeny of placental mammals, the results support the view that humans more recently shared a common ancestor with ungulates than with rodents.