Many reptiles exhibit temperature-dependent sex determination where the incubation temperature of the egg determines the gonadal sex of the individual. If exogenous estrogen is administered during the temperature- sensitive period to embryos incubating at a male-producing temperature, the temperature effects can be overridden and females will be produced. Inhibiting production of endogenous estrogens at female-biased incubation temperatures results in embryos developing as males rather than females. Thus, estrogen-estrogen receptor-dependent mechanisms appear to play a key role in female sex determination. The present study characterized the expression of the estrogen receptor during the critical period of temperature sensitivity in the red-eared slider turtle, Trachemys scripta. Polymerase chain reaction was used to amplify estrogen receptor cDNA. A portion of the estrogen receptor cDNA was used to produce probes for in situ hybridization analyses to localize and quantitate levels of estrogen receptor mRNA at different stages of development in embryos from different incubation temperatures. Estrogen receptor mRNA is expressed in the gonadal tissues of both putative males and putative females even before the gonads begin to resolve as ovaries or testes. There is a greater abundance of estrogen receptor mRNA in putative females at the baginning of the temperature- sensitive period as compared to putative males. In embryos from a female- producing incubation temperature, levels of estrogen receptor mRNA are higher in the beginning of the temperature-sensitive window compared to levels after the ovary is differentiated. These results support the hypothesis that estrogen-estrogen receptor dependent processes are important during sex determination and gonadal differentiation in temperature-dependent sex determination.