The vertebrate Six family of transcription factor genes are homologues of the fruitfly gene sine oculis (so). Human, murine, avian and fish (medaka, zebrafish) homologues have recently been cloned. We report the cloning and developmental pattern of expression of xSix3, the Xenopus laevis homologue of Six3. In addition, we have compared all the known sequences of vertebrate Six3 genes. xSix3 is very homologous to Six3 in other vertebrates in terms of amino acid sequence. The reported developmental pattern of expression of Six3 in chick and mouse includes not only the developing eyes and the ventral diencephalic tissue between them, but also a large, sagittally-oriented telencephalic region. The distribution of xSix3, however, is virtually restricted to the eyes and ventral diencephalon, showing only a very small territory of expression in the telencephalon.