We show here that a zebrafish orthologue of the Thyroid Transcription Factor-1 (TTF-1), nk2.1a, is expressed in the developing thyroid gland. Using a fate mapping approach we found that an early nk2.1a expression domain in the endoderm adjacent to the heart follows morphogenetic movements of the lower jaw, ending up in the region in which the mature thyroid gland is located. We therefore suggest that nk2.1a labels the thyroid precursor cells from somitogenesis stages onwards.