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The population status of the Chatham Island tomtit (Petroica macrocephala chathamensis) was determined for each island of the Chathams group, east of New Zealand. Also, the breeding biology of the population on Rangatira (South East Island), which is free of introduced mammalian pests, was determined from observations made during 8 breeding seasons, 1981/82 to 1988/89. The total population of the Chatham Island tomtit is estimated to be < 1000 birds: Chatham, extinct; Pitt, c. 500; Rangatira, 200-300; Mangere, 70-100; Tapuaenuku (Little Mangere Island), occasional vagrant. Regeneration of scrub and forest habitats on 3 islands is likely to lead to gradual increases in the tomtit populations there. The nesting season on Rangatira was from late September to late January, which was just sufficient time for a pair to rear 2 broods successfully. Of 378 nests, 43% were in tangles of pohuehue (Muehlenbeckia australis) vines, 16% in cavities, 12% on a branch, trunk, or stump covered in vines, and for 21% the site was not indicated. The mean height of nests was 2.7 m, and the mean duration of the pre-laying period was 5.9 days. Mean clutch size was 3.1 eggs, and incubation usually started on the day the last egg was laid (82%). Only females were seen incubating, with males feeding their mates at regular intervals. Of 97 eggs, 83% hatched, and 93% of 15 nesting attempts resulted in at least 1 fledgling each. The high nesting success, in comparison to that of mainland populations, is attributed to the absence of mammalian predators on Rangatira. Although our study provided much information for the early stages of the nesting cycle, few data are available for other aspects of the Chatham Island tomtit’s breeding biology, such as length of incubation, and nestling and fledgling periods.