In situ hybridization and immunocytochemical techniques for aromatase localizat ion in each region of brain and gonads in the experimental group( treated with 17B-E2 diet) and control group ( w ithout E2 treatment ) of young grey mullet s were carried out. The results show that aromatase transcript and specific protein were abundant in the telencephalon ( olfactory bulb and cerebrum) , dienecephalon, mesencephalon and cerebelli of young grey mullets. The distributive density of aromatase immunoreact ive cells in each region of brain had obvious dif ference in the sex undifferent iated period between experimental group and control group. In olfactory bulb, the distributive density of aromatase immunoreactive cells in the contro l group was higher than that in experimental group, but the density of positive cells in the dienecephalon, mesencephalon, and cerebelli in the experimental group were about 1- 3 times higher than that in the control group, and the density of aromatase immunoreactive cells showed dominance in the preoptic area of hypothalamus, especially. This suggests that aromatase may play a key role in the sex different iation of young grey mullets. In addition, after sex differentiation aromatase immunoreactivity was localized in the granular cells of ovary and Leydig cells and Sertoli cells of testis. At the same t ime, aromatase immunopositive substance also distributed in the germ cells of ovary and testis. These results revealed that the mechanism of 17 B-E2 inducing feminization may be mediated by aromatase. The study will provide for the first time a new proof . Finally , the physiological role of aromatase in the gonadal development was discussed in the paper.