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Many data suggest an involvement of estrogens in gonadal differentiation in reptiles with temperature-dependent sex determination (TSD). However, the site of estrogen synthesis in two species of freshwater turtles is unclear. In Emys orbicularis , estrogens were shown to be produced by the gonads, whereas in Trachemys scripta, gonadal steroids were not detected. The marine turtle Dermochelys coriacea exhibits TSD but in gonadal development, ovarian differentiation is delayed. Gonadal aromatase activity and estrogen content in this species were measured in embryos incubated at 27° and in embryos incubated at 30.5°, respectively, masculinizing and feminizing temperatures within the range of temperatures found in natural nests. At all stages studied, aromatase activity was present and found to be higher at 30.5° than at 27°. Estrogens were only found at 30.5°. The effects of temperature shifts on gonadal …