Guppy fry were treated for the first 40 days of life with 0, 20, 40, 60 & 100$mu extrm{g}$ of estradiol per gram of food in order to change the sex of normal males to functional females(genetic male). The present investigation deals with the effects of steroid hormones, such as $eta$-estradiol and testosterone, on the sex differentiation in guppy and tilapia. The results obtained were summarized as follows. 1. In B (20$mu extrm{g}$/g diet) group 17$eta$-estradiol-treated, 67.8% of male offsprings were produced. 2. In D (60$mu extrm{g}$/g diet) group 17$eta$-treated, 67% of female offsprings were produced. 3. B, D groups of genetic male brooders had significantly different effects (P<0.01) upon sex ratios of their progeny. 4. This strongly indicates that sex direction has been achieved and that the male is the heterogametic sex. 5. The group that produced the highest percentage of male offspring(male percentage of observed number to expected number was 91%) contained only full-sibling male brooders to the sex-reversed female brooders. 6. After 7 months following treatment, the sex-reversed males had ovarian portion in the anterior region and a testicular portion in the posterior region of the same intersexual gonad, respectively. 7. At 7 months after treatment, the ovareis revealed a complete arrest of the ovarian formation, and appearances of spermatogenetic cell cysts among surviving auxocytes. 8. In most of sex-reversed fish, anterior portion of test is was devoid of sperm ducts including the seminal vesicle and vas deferens. 9. The male transferrin showed two strong bands, while the female transferrin showed a single weak band. 10. One of the two bands of male transferrin showed the same mobility with band of female transferrin.