This work analyses the influence that nest site characteristics have over natural incubation in freshwater turtles Podocnemis expansa and Podocnemis unifilis. Results indicate that neither carapace size nor weight of P. expansa hatchlings depends on environmental characteristics. This suggests that incubation occurs in a homogeneous environment in terms of moisture. On the other hand, embryonic development of P. unifilis is affected by soil moisture, and nests located in very humid beach areas generate smaller hatchlings than nests incubated in drier areas.