Abstract 1. 1. The water loss from naturally incubated eggs of the Wedge-tailed Shearwater was measured, together with the water-vapor pressure of the incubated egg's micro-climate, the egg temperature, the water-vapor conductance of the shell and other characteristics of the shell. 2. 2. The mean daily water loss (. M H 2 O ) from the eggs (154.6 mg/day) was only 49.8% of the value expected on the basis of the mass of the freshly-laid egg. The low value for M H 2 o was attributed largely to a low water-vapor conductance ( G H 2 O ) of the shell (56.9% of predictions). The low G H 2 O . in turn, was due to fewer pores in the shell. 3. 3. Egg temperature (35.0 C) was lower than that of the eggs of birds in general, and this was considered to be a contributory factor to the low daily water loss. 4. 4. The water-vapor pressure ( P H 2O nest) of the egg's microclimate (19.57 torr) was similar to that of many surface-nesting birds in spite of the burrow-nesting habits of the shearwaters. 5. 5. It was calculated that the total pre-pippiny water loss from the eggs represented 12.2% of the initial mass of egg. 6. 6. It was concluded that prolonged incubation in the Wedge-tailed Shearwater, as in other smaller Procellariiformes. is associated with reduced values for . M H 2O and G H 2O and fewer pores in the shell.