This paper is a study of sex ratios among turtle (Emys orbicularis) hatchlings and young from eggs incubated in the ground, as a function of soil temperature during the gonadal thermosensitive period. Five experiments were performed in Brenne (France) during the summers of 1974, 1975, and 1979. In all these experiments, the sex ratios were found to be skewed. In one experiment, the percentage of males was nearly 50%, whereas the number of females was reduced and the other individuals displayed an intersexual phenotype (the gonads were ovotestes). In the other experiments, the number of males was reduced, whereas the number of females approached 50% or was superior to 50% and the other animals displayed an intersexual (ovotestes) or indeterminate (thin atypical gonads) phenotype. The soil temperature was registered at the same depth as the eggs, and the temperature curves were analyzed for the 12 days following the beginning of the gonadal thermosensitive period (stage 16 of embryonic development). This analysis shows that the first half of this period is the most important for gonadal sexual differentiation. In the first experiment, temperature remained below or surpassed slightly the threshold temperature of 28.5°C (at which males and females, but also intersexes, are obtained). These conditions favored male differentiation. Then, the maxima were higher, allowing female differentiation which was either typical (2 cases of 21 only) or partial (intersexes which are probably genetic females). In the other experiments, the maxima were high, reaching 35 to 40°C, from the very beginning of the thermosensitive period, and although alternating with low minima (inferior to 25°C) they favored female differentiation and inhibited male differentiation. Therefore, intersexes, indeterminates, and probably some phenotypic females would correspond to genetic males.