Two experiments were carried out to clarify the effects of thyroxine (T4) and triiodothyronine (T3) on flounder metamorphosis. In the first experiment, three doses of T4 were administered to premetamorphic flounder larvae together with thiourea (TU). Thiourea treatment (30 ppm in ambient seawater) alone caused metamorphic stasis in the fish, although the beginning of the right eye migration was not completely inhibited. T4 at 100 and 10 ppb in ambient seawater induced metamorphosis in the thiourea-treated larvae. The fish of these groups completed translocation of the right eye to the left side and resorption of elongated dorsal fin rays. However, 1 ppb T4 failed to induce metamorphic climax. The elongated dorsal fin rays continued to grow until the end of the experiment in the group treated with TU alone and in combination with 1 ppb T4. In the second experiment, premetamorphic flounder larvae were treated with two doses of T4 and three doses of T3. The T3 treatment was several times more potent than T4 treatment in the induction of metamorphosis. The results of the present study suggest that the metamorphic events of flounder larvae are dependent on the thyroid hormone concentration, and the relative biological potency of T4 and T3 on flounder metamorphosis is similar to that observed in mammals and amphibia.