The normal differentiation of the gonads of Tilapia aurea is described. The process is similar to that described for other teleosteans and differs only in a few points. The appearance of the 1st sign of the gonad is delayed and takes place 10 days after hatching. The path of migration of the gonocytes to the gonadal anlagen in the anterior part of the fish is on the splanchnopleura. Only at the caudal quarter of the gland, the path of migration is the lateral part of the somatopleura. Estrogens in concentrations of 50 and 100 [mu]g/1 of aquarium water, for a period of 3-4 weeks at the time of gonadal differentiation, bring about the destruction of the gonads of Tilapia. Androgens at a concentration as high as 1.0 mg/l have only a variable effect on the destruction of the gonad.