Sex determination is controlled either by genetic or environmental factors. In mammals Sry initiates determination but no homologue of this gene exists in non-mammalian species. Other genes of the mammalian sex-determining pathway have been identified in gonads of different vertebrates. Sox9, Dax1, and Dmrt1 are expressed at the onset of gonadal development in birds and reptiles. In the sea turtle Lepidochelys olivacea, a species with temperature sex determination (TSD), Sox9 is expressed in undifferentiated gonads at male- (MPT) or female-promoting temperatures (FPT). At MPT, Sox9 remains expressed in male gonads, but at FPT it is downregulated coinciding with the onset of the ovarian morphologic differentiation and female sex determination. At MPT however, male sex is determined early than at FPT in still undifferentiated gonads suggesting that other genes maintain Sox9 expression in testis. Here we used RT-PCR to study the expression profiles of Dax1, Dmrt1, and Sox9 in gonads of embryos of L. olivacea incubated at MPT or at FPT. The profiles were correlated with sex determination during and after the temperature-sensitive period (TSP). Dax1 maintained similar levels at both temperatures during the TSP. The Dax1 expression level increased significantly in ovaries compared to testes at stage 27, once they were morphologically distinct. The expression levels of Dmrt1 were higher at MPT than at FPT at all stages, in contrast with Sox9 levels which were similar at both temperatures at stages 23-25. Together, current results suggest that, whereas Dax1 is not involved in TSD in L. olivacea, upregulation of Dmrt1 and downregulation of Sox9 may play a role in male and female sex determination, respectively.