Temperature-dependent sex determination has previously been reported for the flatback turtle (Natator depressus). The present study investigates the thermosensitive embryonic developmental stages for the sexual differentiation of this species. Groups of eggs incubated initially at constant temperatures of 26, 29 and 32 C were shifted once during incubation from either a constant masculinising temperature (26 or 29 C) to a constant feminising temperature (32 C) or vice versa. Findings from this study support the hypothesis that the effect of temperature and the timing of the thermosensitive period are dependent upon the specific temperature utilised during incubation. The thermosensitive developmental stages at masculinising temperatures were different to those of feminising temperatures. For the 26 to 32C temperature shift, the thermosensitive period was confined to a single developmental stage (Stage 24). Ovarian development was determined at a later developmental stage than testicular development.