The Scarlet Ibis, Eudocimus ruber (Linnaeus), fam. Threskiornithidae, is the closest relative of the White Ibis, Eudocimus albus (Linnaeus). The two species live in adjoining geographical areas. Nothing is known about the Scarlet Ibis’s breeding biology in the wild and only little about its breeding biology in captivity. A better understanding of the breeding of this species in captivity should help to improve its management and also to elucidate its relationship with its much better known counterpart, the White Ibis. Three types of display behaviour are recognized in courtship: the snap display, the dipping snap display and twig pulling. The latter occurs also in greeting ceremonies, together with the up-down display, and as a precopulatory activity. Breeding pairs are stable for one breeding cycle, but occasionally promiscuous matings can be observed. Of 16 birds breeding in 1982, 12 bred again in 1983, all with a different partner. Egg laying is preceded by a period of increasing appearance of both members of the couple together on the nest, where copulation takes place. Nests contain 2-3 eggs, which are incubated for 21-23 days. Hatching is indicated by a sudden decrease in incubation. Hatching failure can be succeeded by the laying of replacement clutches. Nesting failures were due to destruction of nests or young, often resulting from disturbances by humans or domestic cats. One pair incubated for over 4 months without apparent interruption, probably without eggs being present for most of the time. Females spend more time incubating than males. Nest reliefs occur at a much higher rate (ca. 1 per hour) than in the wild (ca. 2 per day). For up to 3 weeks after hatching, one parent bird stays at the nest almost continuously. Unattended nests are quickly visited by other birds, predominantly males. One parent, mostly the male, almost immediately chases an intruder away. Slight variations in breeding behaviour could not be correlated with breeding success. A dominance hierarchy could be distinguished between adult birds. Aggressive encounters consisted of three different behaviour patterns: fighting, forward threat and stab-and-counter stab. Often one bird gave way to another without overt signs of aggression. The hierarchy was not linear but could be subdivided into three groupings. The status of a bird was sex-dependent, correlated with weight and length of the bill, but not with nest-ownership or age. The breeding behaviour of the Scarlet Ibis and the White Ibis seems to be so similar that, bearing in mind their similarity in morphology, the question may be posed whether they are conspecific.