SUMMARYROBERTSON, H.A. and M.D. DENNISON. 1984. Sexual dimorphism of the Chatham Island WarblerGerygone albofrontata.Emu 84: 103–107.Evidence that the Chatham Island Warbler is sexually dimorphic in plumage and size is presented from field observations on breeding pairs on three islands in the Chatham Group, New Zealand, over five breeding seasons; and from an examination of museum skins. Descriptions, measurements, and distinguishing field and behavioural characteristics of adult males and females, and juveniles are given. It is suggested thatG. albofrontatais not a derivative ofG. igata, but the result of an earlier invasion by a warbler that has since become extinct on the New Zealand mainland.