Carettochelys insculpta lays its eggs in shallow nests constructed in clean fine sand adjacent to water. Six nests had the following thermal characteristics: (1) core temperatures ranging from 26.1 to 33.7°C early in September, from 30.0 to 38.7°C early in October and from 28.4 to 36.8°C early in November; (2) a pronounced daily cycle in core temperatures of up to 6.4°C in amplitude, with the minimum occurring after sunrise in mid.morning and the maximum occurring in the evening; (3) a difference in the temperatures experienced by the top and bottom eggs in a single nest of up to 3.5°C at any one time. Nest heating and the maximum temperatures achieved were principally driven by solar irradiation, not ambient air temperatures. Hot nests produced females exclusively, nests intermediate in temperature produced a mixture of sexes, and the sole cool nest produced males exclusively. In nests that produced both sexes, males emerged from the deepest coolest eggs whereas females emerged from the shallowest eggs. Hence, field nests may yield both sexes either because egg temperatures are on the sex.determining threshold or because thermal gradients in nests span both male. and female.producing temperatures.