This is the first study on the effects of salinity as a stress on the differential expression of the aromatases (CYP19 a & b) in the Nile tilapia (Oreochromis niloticus) tissues (ovary, testis and brain of female). The experiment was conducted to study the effects of five different salinity levels (10‰, 15‰, 20‰, 25‰ and 30‰) as a stress on the expression of aromatase genes. The fish were transferred gradually from lower concentration to higher concentration of salinity for adaptation. The results of the expression of CYP19a in the gonads of O. niloticus as a response to the stress of salinity revealed the positive correlation between salinity and the relative CYP19a expression in the ovaries and testes of all treatments with marked higher values in ovaries than testes and this can be ranked in the order: 30‰ > 25‰ > 20‰ > 15‰ > 10‰. With the increase of salinity levels, the relative CYP19a expression in the ovaries varied from 0.8 with the treatment of 10‰ to 7.58 with that of 30‰, while the expression in the testes ranged from 0.65 to 3.48 respectively. Similarly, the direct relationship was observed between the relative expression of CYP19b in the brain of females (0.026 to 0.037) and the increasing levels of salinity.