SUMMARY Thyroid-mediated otolith growth in tilapia was evaluated by the ontogenic triiodothyronine (T 3 ) profile revealed by radioimmunoassay during the first month after hatching. Thyroid hormone receptor genes( TR α and TR β) were cloned and only the expression of TR α mRNA, quantified by real-time PCR, was similar to the T 3 profile. Variations in otolith growth showed median correlation with the T 3 profile and TR α mRNA expression pattern. Hypothyroidism and hyperthyroidism were induced in tilapia juveniles and larvae by administration of different concentrations of thiourea (TU) and T 3 , respectively, for 13 days. T 3 and TU had little effect on otolith growth during the larval stage. However, T 3 increased otolith growth and TU retarded, or stopped, otolith growth during the juvenile stage. Furthermore, TU treatment caused permanent changes in otolith shape in the ventral area. Otolith growth recovered slowly from hypothyroidism, requiring 2 days to form an increment during the first week. These results suggest that otolith growth, at least during the juvenile stage,is regulated by the thyroid hormones and the process may be mediated by TR α.