A relatively uncommon but intriguing form of bird song, the significance of which is imperfectly understood, is synchronized duetting between a mated male and female. Most reports of birds engaging in this come from the tropics, with examples from Central America (Skutch, 1940), South America (Haverschmidt, 1947), Africa (Moreau, 1941; Thorpe, 1963; Grimes, 1966), Madagascar (van Someren, 1947), and Malaya (Young, 1942). A few examples from the temperate zone have also come to light among owls, Himalayan (Osmaston, 1941) and Australian (Robinson, 1947: 14-17) passerines, and the Canada Goose (Branta canadensis) (Collias and Jahn, 1959), Carolina Wren (Thryothorus ludovicianus), and three species of quail (Stokes and Williams, 1968) of North America. A selected list of species may be found in Van Tyne and Berger (1959: 140). The present paper is an analysis of all instances known to us of duetting by New Guinea birds, based largely on our previously unpublished field observations. As it has been suggested that the incidence of duetting may correlate with type of habitat, possibilities for visual contact between singers, or sexual dimorphism of plumage, these points are considered in each species discussed. Duets may be divided conveniently into three categories. (1) The leader (generally the male) sings one phrase, and at its conclusion the second bird (generally the female) sings another phrase. Often the follower begins so precisely upon cessation of the leader's phrase that the result may easily pass for a conventional single song, unless the observer can see the singers or is close enough to hear that different notes come from different places. For example, the Marbled Wood-Quail (Odontophorus gujanensis) calls a rapidly repeated corcorovado, in which corcoro is always sung by one member of a pair and vado by the other (Chapman, 1929: 275; Armstrong, 1963). The term antiphonal singing has been used for this type of duetting, in which male and female sing alternately rather than simultaneously. In this category belong the Central American wrens described by Skutch (1940) and Amaurornis olivaceus, Pitohui kirhocephalus, and Philemon novaeguineae (see below) of New Guinea. (2) Members of a pair sing different phrases simultaneously. The synchronization between the two phrases may nevertheless be very exact; the female may begin only a fraction of a second after the male. In New Guinea the duets of Megapodius freycinet, Campochaera sloetii, and Coracina montana are of this type. (3) The male and female sing virtually identical phrases in unison. Such unison duets are practiced by Centropus toulou (Cuculidae) of Madagascar (van Someren, 1947), Aspatha gularis (Momotidae) of Central