Samples of eggs laid by nine painted turtles (Chrysemys picta) were half buried in substrates differing in water potential and incubated at 29 C. Other eggs from these clutches were incubated on wire platforms above the same substrates. Eggs on the wire platforms declined in mass between the outset of study and hatching owing to a net outward diffusion of water vapor, with eggs held above relatively dry substrates losing more vapor than eggs held above relatively wet substrates. Eggs half buried in the substrates increased in mass during the first half of incubation by amounts that were directly related to substrate water potential, but decreased in mass during the second half of incubation as the net flux of water shifted from inward to dutward. The conductance of painted turtle eggs to water vapor was 70 times higher than expected for avian eggs of comparable mass, yet transpirational loss of water from incubating turtle eggs nonetheless was small owing to the very small gradient in vapor pressure between eggs and air trapped inside the containers. Eggs exposed to wet substrates had longer incubation periods and higher hatching success than eggs exposed to drier substrates. Moreover, turtles hatching from eggs exposed to relatively wet conditions were larger than hatchlings emerging from eggs incubated in slightly drier settings.