The alar plate of the prosencephalon differentiates into a tectum-like structure when transplanted and integrated into the mesencephalon around 10-somite stage. Homeobox containing gene, en, is expressed in the mesencephalon with caudorostral gradient from 7-somite stage. Here expression of en in the prosencephalic transplant in the mesencephalon was studied immunohistochemically with anti-en antibody. Transplantation was carried out between quail and chick. It was shown that the transplant was stained with anti-en antibody at the caudal part of the mesencephalon. At the rostral part, the transplant did not express en. When the transplant was not contiguous to the caudal part of the host mesencephalon, it did not express en. The present study suggests that the prosencephalic transplant differentiates into the tectum-like structure when it expresses en at the caudal part of the mesencephalon.