The development of the tectum and diencephalon in Xenopus has been investigated in relation to recent descriptions of the establishment of the retinotectal projection. Tritiated thymidine autoradiography and bromodeoxyuridine immunohistology were used to identify the stages at which cells became postmitotic. Cells in the diencephalon were found to become postmitotic before cells in the tectum. At the time of arrival of the first optic fibres (stage 37/38) no postmitotic cells appeared to be present in the tectal precursor region. The first postmitotic cells which could be definitely assigned to the tectum appeared between stages 41 and 45. The results suggest that the initial retinotopic ordering of optic fibres observed from stage 37/38 relates to the position of fibres in the optic tract and not the tectum.