Subsurface temperature variation suggests that the thermal environment of giant Amazon river turtle (Podocnemis expansa) nests is influenced by the proximity of the water table and by the nest depth. The difference between vertical temperature gradients along fluvial beaches indicates that nest placement can exert a significant influence on substrate temperature. Temperatures decrease significantly with depth where P. expansa nest on the beach, but do not change with depth in lower beach areas where P. unifilis nest. Daily temperature variation decreases with depth along the entire beach.