The size and the number of thyroid masses in medium larval, climax, and postmetamorphic developmental stages ofPleurodeles waltlwere examined in histological sections. Morphological data were also obtained from postmetamorphic specimens, that had been treated for a year with thiourea. In addition to a pair of lateral thyroid glands that developed during larval stages, microscopic examination revealed the presence of two or three supernumerary thyroid glands that were very small in the climax stage, but increased appreciably in size and development in 1‐year‐old postmetamorphic animals. Structural modifications in the supernumerary thyroids similar to those observed in the lateral thyroid glands were observed in thiourea‐exposed, postmetamorphic animals. These super numerary thyroids can be classified as one or two thyroid glands located in the anterior medial sagittal region of the lower jaw, and one posterior thyroid gland located in a left parasagktal region near the pericardial sac. On the basis of the thyroid control on body development and growth in the postmetamorphic stage, reported in other studies, including our own, and of the postmetamorphic mor phology of the supernumerary thyroid glands inP. waltl, an evolutive adaptation of the thyroid system to the habitat of this species is hypothesized.