A recent study demonstrated that the embryos of soft-shelled turtles can reposition themselves within their eggs to exploit locally warm conditions. In the current paper, we ask whether turtle embryos actively seek out optimal thermal environments for their development, as do post-hatching individuals. Specifically, (1) do reptile embryos move away from dangerously-high temperatures, as well as towards warm temperatures? and (2) is such embryonic movement due to active thermoregulation, or (more simply) to passive embryonic repositioning caused by local heat-induced changes in viscosity of fluids within the egg? Our experiments with an emydid turtle (Chinemys reevesii) show that embryos avoid dangerously high temperatures by moving to cooler regions of the egg. The repositioning of embryos is an active not passive process: live embryos move toward a heat source, whereas dead ones do not. Overall, our results suggest that behavioural thermoregulation by turtle embryos is an active process, genuinely analogous to the thermoregulatory behaviour exhibited by post-hatching ectotherms.