The role of thyroid hormone in flounder metamorphosis was studied by use of thyroxine and the antithyroidal agent, thiourea (TU). T4 stimulated metamorphosis of pelagic larvae, producing miniatures of naturally metamorphosed benthic juveniles. In contrast, TU induced metamorphic stasis, resulting in giant pelagic larvae. These results indicate that the thyroid plays a role in flounder metamorphosis comparable to its well-known developmental role in amphibian metamorphosis.