The natural incubation period of Great Frigatebird (Fregata minor) eggs reported in the literature (55 days) was 74% longer than the value predicted from initial egg mass (89.1 g). The long incubation period was associated with a low daily water loss (213.5 mg/day) from the eggs and few pores in the eggshell (4,643 pores/egg). The initial event in the pipping process, which represented 7.2% of the duration of the incubation period but accounted for 24.9% of the total water loss from the egg, was a star-shaped fracture of the shell which increased water loss from the egg and persisted for several days before hatching was complete. These characteristics of the eggs of the Great Frigatebird are contrasted with those from the Red-footed Booby (Sula sula), the only other altricial, pelecaniform seabird with a long incubation period, for which data are available.