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Observations on the diving petrel (Pelecanoides u. urinatrix) were made on Green Island, Mercury group, and other small islands in New Zealand in 1966 and 1967. The various calls and communications are described and weights and measurements given of adults and of growing chicks. Various behaviour patterns are briefly compared with the northern alcid, the Cassin auklet (Ptychoramphus aleutica). Incubation was recorded by assistants for one egg at 53 days. Chicks reach the adult weight by 30 days of age and attain the average at rest body temperature of the adult about the same time. Daily growth rates of the various body parts, total length, closed wing, tarsus, toe, culmen and tail are given for developing chicks. Fluctuations in incubation temperature of two eggs are given. Other eggs showed considerable tolerance to lowered temperatures. The difference between at rest and active body temperatures of adults is indicated at approximately 2°C. The adults arrive at the breeding site early in April with a peak of activity by the end of May. The egg-laying period is calculated to begin early in August and last for about one month. Average body weights of 20 adults in April were about 18 grams more than in June and October.