The transposable element mariner occurs widely in the melanogaster species group of Drosophila. However, in drosophilids outside of the melanogaster species group, sequences showing strong DNA hybridization with mariner are found only in the genus Zaprionus. The mariner sequence obtained from Zaprionus tuberculatus is 97% identical with that from Drosophila mauritiana, a member of the melanogaster species subgroup, whereas a mariner sequence isolated from Drosophila tsacasi is only 92% identical with that from D. mauritiana. Because D. tsacasi is much more closely related to D. mauritiana than is Zaprionus, the presence of mariner in Zaprionus may result from horizontal transfer. In order to confirm lack of a close phylogenetic relationship between the genus Zaprionus and the melanogaster species group, we compared the alcohol dehydrogenase (Adh) sequences among these species. The results show that the coding region of Adh is only 82% identical between Z. tuberculatus and D. mauritiana, as compared with 90% identical between D. tsacasi and D. mauritiana. Furthermore, the mariner gene phylogeny obtained by maximum likelihood and maximum parsimony analyses is discordant with the species phylogeny estimated by using the Adh genes. The only inconsistency in the mariner gene phylogeny is in the placement of the Zaprionus mariner sequence, which clusters with mariner from Drosophila teissieri and Drosophila yakuba in the melanogaster species subgroup. These results strongly suggest horizontal transfer.