As in all other lower vertebrates, sex differentiation in fish is susceptible to environmental and steroidogenic stimulation or inhibition. Alteration of the sex differentiation process is possible in many fishes through the manipulation of the environment and steroid function. Some chemicals, which have androgenic or estrogenic function, are known to alter the gonadal sex in fish from female to male or vice versa if administered during the period of gonadal sex differentiation. Other than direct androgenic or estrogenic effects, there are chemicals which are capable of disrupting the enzymatic pathway of natural androgenic-estrogenic balance in fish. Among the non-steroidal aromatase inhibitors (AI), letrozole is typically used as an effective human drug in the treatment of estrogen-dependent disease, including breast cancer, and was found to be effective in suppression of all estrogens after oral administration. )]. Gonadal development was observed histologically using a microscope, and the expression of CYP19A and CYP19A was expressed in female, and A quantitative real-time PCR analysis revealed that the expression of the gene in female was significantly inhibited after treatment with LE. Though expression of was higher at 22 days post hatch (dph) than at 9 dph, it was only 2.11 the level of the control female. gene expression was up-regulated gradually in LE treated female fry during sex differentiation, and reached the same level as control males at 150 dph. In LE treated female fry, degeneration of the ovarian cavity and primary spermatocytes were observed in sections of the gonad. The functional testes were developed at 150 dph. Our research demonstrates that aromatase activity is necessary for ovarian formation and development during sex differentiation in T. obscures during sex differentiation.