The changes of 3,5,3'triiodo-L-thyronine (T3) levels during the early developmental stages of orangespotted grouper (Epinephelus coioides) were examined by radioimmunoassay, and the effects of T3 on grouper yolk-sac larvae were studied. The fertilized eggs contained a substantial amount of T3. The levels of T3 in eggs gradually decreased during embryonic and yolksac larval development, and T3 could not be found at first feeding. The decrease of T3 levels in fertilized eggs during development suggests that TH could be utilized to act on the embryonic development and the yolksac larval development of orangespotted grouper. The effect of T3 on larval development was not obvious within 24 hours posthatching(hph), however, the advancement in development of gut was induced by T3 treatment after 24 hph. Compared to the control, the diameter of larval rectum in 0.01 mg·L-1T3 groups was significantly longer but the differences in both the diameter of intestine and the total length of larvae was not significant at 72 hph. Both the diameters of larval intestine and return the rectum in 0.1 mg·L-1 and 1 mg·L-1T3 groups were obviously longer than those in the control, but the total length of larvae was obviously shorter than that in the control at 72 hph. The resorption of yolk sac was accelerated by 1 mg·L-1 of T3 treatment, but that of oil globule was not. The rate of survival in 0.01 mg·L-1T3 groups was obviously higher than that in the control in 8 dph. The results indicated that the action of T3 on development of gut was closely related to developmental stages. The development of larval gut was susceptible to T3 treatment after 24 hph but that was not within 24 hph. The rate of larval survival could be promoted by lower concentration of 0.01 mg·L-1T3 treatment.