Temperature determines the sex of hatchling green turtles (Chelonia mydas) produced from eggs incubated in a beach hatchery under different temperature regimens. Cola and cool nests (less than 28 degrees C) produced almost no females (0 to 10 percent) and warm, thermostable nests (greater than 29.5 degrees C) produced almost all females (95 to 100 percent). A few intersex hatchlings were produced at lower temperatures. Since little concern is given to temperatures at which sea turtle eggs are incubated in artificial hatcheries, present conservation practices may be producing all male, all female, or even intersex hatchlings.