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The breeding cycle of the Westland black petrel (Procellaria westlandica) is outlined from observations made in the 1970 and 1971 breeding seasons and from the literature. Adults arrive at the colonies in late March to excavate and claim burrows, reform pairs and build nests. Egg laying commences in early May and seems to be preceded by a honeymoon period. Most eggs are laid in a short peak period of about three weeks in May. Hatching begins about the third week in July following an incubation period of about 57–65 days. Chicks are brooded for up to two weeks and then spend the rest of their 120+ day fledging period alone except during feeding visits by the parents. Very heavy egg and chick losses were sustained in the two seasons studied and the disappearance of big chicks in September each season suggests that the colonies may be “birded.” The long incubation and fledging periods relative to some other Procellariidae are interpreted as an adaptation to a sparse and variable food supply. Competition for food with summer breeding shearwaters such as P. parkinsoni seems the most plausible explanation for the winter breeding season of P. westlandica.