Recent studies strongly indicate a bidirectional relationship between sexual distinction and sex steroid hormones. In this study, Western blotting and immunohistochemistry were used to estimate the level of estrogen receptor (ER) and androgen receptor (AR) in the liver, gonads, kidney, and intestine of female and male Nile Tilapia, Oreochromis niloticus. The effects of a single injection of 17β-estradiol (E2), testosterone on the gene expression of ER-β and AR-α1 were determined. The expression of ER-β in females was higher than in the male liver and gonad (P<0.05). AR-α1 expression in in males was higher than females in the liver, gonad, and intestine (P<0.05). E2 increased the expression of ER-β in the livers of female fish (P<0.05). Testosterone (T) increased the expression of AR-α in the liver and intestine of male fish (P<0.05). Contrary to the result of AR, a Western blot analysis indicated that the level of ER protein in the female liver and gonad was significantly higher than that of males (P<0.05). Immunopositive results were the highest in the gonad followed by the intestine and kidney. Moreover, the immunolabeling signal was intensified in females but not in males, as shown via Western blot.