We calculated water vapor conductance (a product of eggshell porosity) from the first definitively identified sauropod egg (Megaloolithus patagonicus) from the Auca Mahuevo locality in Argentina. We then compared the results with those from M. siruguei (an egg type long associated with sauropod dinosaurs) from the Pinyes locality in Spain. The 14-cm Auca Mahuevo egg has a thinner eggshell and 47 times fewer pores than the 22-cm M. siruguei specimen. The resulting water vapor conductance (G_H2O) of the titanosaur and M. siruguei eggs is 341 and 3979 mg H₂O day⁻¹ Torr⁻¹, respectively; these values are two and ten times greater than in avian eggs of comparable size, but lower than in eggs of most modern reptiles. Clutches from Auca Mahuevo typically contain 20–40 eggs; in contrast, M. siruguei clutches from the Pinyes site average nine eggs. The G_H2O of M. siruguei exceeds that of the Argentine egg by an order of magnitude, supporting previous inferences of egg burial. The G_H2O of the Argentine titanosaur egg closely approximates that of Troodon and some oviraptorid eggs, previously calculated as equal to or two times greater than, respectively, the G_H2O of avian eggs of similar size. Higher embryonic growth rates (relative to modern reptiles), especially in some dinosaurs with large clutch mass, may have required incubation in a more open environment, where water conservation represented a more critical factor than in a buried clutch. The lower G_H2O calculated for the two megaloolithid eggs is consistent with previous interpretations of nesting mode that are based on site taphonomy and nesting traces. This study indicates that at least some dinosaurs did not fully bury their eggs.