The thyroid hormone levels in female rabbitfish, Siganus guttatus, plasma, eggs, and yolk-sac larvae were measured before and after thyroxine (T₄) injection in female spawners at doses of 1, 10, and 100 μg T₄/g body weight (BW) fish. T₄ and triiodothyronine (T₃) levels in maternal plasma, eggs, and yolk-sac larvae were elevated following T₄ administration. Apparently, there is conversion of T₄ into T₃ in the broodfish which suggests the presence of the enzyme, 5′-monodeiodinase, in rabbitfish. T₄ and T₃ in maternal circulation were easily transferred into the oocytes and subsequently into the larvae. Larvae from spawners treated with 10 and 100 μg T₄/g BW tended to be longer and showed sornewhat better survival compared to the control and those injected with 1 μg T₄/g BW 7 days after hatching. These findings suggest that thyroid hormones may play an important role during early larval development of rabbitfish.