We studied the New Zealand morepork (Ninox novaeseelandiae) over 2 breeding seasons on Mokoia I, Lake Rotorua, North Island, New Zealand. Ten pairs were monitored in the 1995/96 breeding season and 8 in the 1996/97 season. Nest sites included tree cavities, hollows amongst tree fern fronds, nest boxes provided for saddleback (Philesturnus carunculatus) and scrapes on the groung. Nest cavities were 0-5.2 m agl. Clutch size was 1-3 eggs; egg dimensions averaged 39.0 mm × 32.9 mm. The incubation period for 1 clutch was at least 24 days. Only females were observed to incubate eggs and brood nestlings; males roosted nearby. Two chicks were weighed and measured throughout their development and the nestling period was determined for 1 chick. Nestling development is described. Breeding success was lower in the year after a poisoning operation to eradicate mice from the island. Juvenile mortality was high after fledging. The dispersal of 3 juveniles was monitored, and females appeared to move earlier and disperse farther than males.