The breeding of the Chatham Island warbler (Gerygone albofrontata) was studied over five seasons on three islands in the Chatham Island Group. The breeding season is short, and only one brood is raised per year. On predator-free ‘petrel islands’, nests were low to the ground in dense vegetation, whereas on Chatham Island nests were high and in the open. Mean clutch size was 3.1 eggs (n=79). Incubation and nestling periods were both about 20 days. Density of breeding birds was highest in regenerating forest clumps on predator-free islands, with about 10 pairs per hectare. Comparisons are made with the breeding biology of the Grey Warbler (G. igata) of the New Zealand mainland and with other Gerygone species. Brood parasitism by the Shining Cuckoo (Chrysococcyx lucidus) and how vulnerable the Chatham Island Warbler is to extinction are discussed.