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Rigid-shelled eggs of softshell turtles (Trionyx spiniferus) incubated at 29d̀C under hydric conditions simulating those at the interior of natural nests (i.e. eggs not contacting the substrate) declined in mass during incubation owing to the continuous escape of water vapour to air trapped inside the chambers, with eggs incubated in relatively dry chambers declining more in mass than eggs held in relatively wet chambers. Eggs held under hydric conditions approximating those at the periphery of natural nests (i.e. eggs partly buried in the substrate) also declined in mass during incubation, but at lower rates than characterized eggs not contacting the substrate, indicating that absorption of liquid water from the substrate compensated for part of the water vapour escaping to the air inside the chambers. There was no relationship between hatching success and either the position of eggs in the chambers or the wetness of substrates, nor was there any variation among experimental groups in the size of hatchlings. Thus, development of embryos of Trionyx spiniferus seems not to be closely coupled with the hydric environment of the nest, a situation contrasting with that of embryos of those species of turtles producing pliable-shelled eggs.