In many species of reptiles, sex is determined at fertilization by zygotic sex chromosome composition. In other species, including all crocodilians, most turtles and some lizards, sex is determined by temperature during the earlier stages of gonadal differentiation. The effects of exogenous estrogens, antiestrogens and aromatase inhibitors at different temperatures have unambiguously demonstrated the involvement of estrogens in sexual differentiation of the gonads. Aromatase is the enzyme that converts androgens to estrogens. Gonadal aromatase activity is well correlated with gonadal structure. It increases exponentially in differentiating ovaries, whereas it remains low in differentiating testes. Moreover, there is a high correlation between the thermosensitive periods for both ovary differentiation and increase in aromatase activity. We suggest that a thermosensitive factor intervenes, directly or indirectly, in the transcriptional regulation of the aromatase gene in reptiles with temperature-dependent sex determination.