SUMMARYWINGHAM, E.J. 1984. Breeding biology of the Australasian GannetMorus senator(Gray) at Motu Karamarama, Hauraki Gulf, New Zealand I. The egg. Emu 84: 129–136.Breeding biology of the Australasian Gannet was studied over three breeding seasons (1978–80). The nest sites and density, first date and duration of egg-laying, median date of laying, clutch size and replacement laying, incubation period and egg dimensions, were measured. Similar information is available for the Atlantic Gannet and the Cape Gannet, allowing comparisons between the three Gannets to be made.The Australasian Gannet varies more in the first and median dates of laying than the Atlantic Gannet. This is thought to be related to weather and food supply. Most Australasian Gannet eggs that are lost are lost and replaced after three- quarters of the spread of laying has elapsed. In the Atlantic Gannet eggs are mostly lost and replaced before the mid-laying date. The Australasian Gannet has a proportionally larger egg than the Atlantic Gannet. These differences probably result from the Australasian Gannet experiencing a less seasonal and predictable environment than the Atlantic Gannet.