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¹ Universidad del Papaloapan (UNPA), Laboratorio de Acuicultura, Des: Ciencias Agropecuarias. Av. Ferrocarril s/n, Col. Ciudad Universitaria, C.P. 68400, Loma Bonita, Mexico DOI : 10.4194/1303-2712-v17_3_16 Viewed : 3719 - Downloaded : 3517 Production of YY-males is one of the most important alternatives to the commercial use of hormones for producing monosex populations in the culture of Nile tilapia. One of the final stages of YY-technology is the feminization of the progeny of XY-females in order to obtain YY-females, which when crossed with YY-males makes possible to obtain populations composed of 100% YY-males. The aim of the study was to evaluate the effects of four concentrations (100, 200, 300, 400 mg kg^-1) of estrogen 17α-ethinylestradiol (EE₂) on the sex ratio, growth, condition factor (CV) and gonadosomatic index (GSI) of the progeny of XY-females. Significant differences were observed in wet weight, total length and CV during the experiment; however, final values in EE₂-treated groups showed not significant differences with control group. The observed proportion of females deviated significantly from the proportion of females expected in the progeny of XY-females in all groups fed EE₂; however, only 90% feminization was attained. The group fed 200 mg kg^-1 of EE₂ showed a significantly higher value of GSI in comparison to that observed in the control group. By increasing the concentration of EE₂ it is possible to increase the proportion of females without affecting growth or GSI. Keywords : Sex-reversal, YY-females, gonadal development, YY-males