In the Llanos (Flatlands) of Venezuela a high malaria parasite rate was found in nesting birds, mainly in many species of Ciconiiformes, in contrast to a very low one in adults. Species of Plasmodium in birds of this order have seldom been reported. The high densities and sporozoite rates of the local vector, Aedeomyia squamipennis, and the increasing parasite rates with age, suggest a great intensity of transmission, leading to 100% infection by the time the young birds leave their nests. The situation is regarded as a form of holoendemicity equivalent to that described in man in certain parts of Africa and elsewhere, which also produces a high mortality and is probably the cause of population control. Double and triple infections of different subgenera of Plasmodium were extremely frequent, and probably also of different species in the same sub-genus. Such mixed infections with high densities of parasites suggest the possibility of hybridization, and the presence of hybrids may explain the difficulties in the identification of many of the isolates.