This report is based on review of literature and examination of a great number of blood smears from native birds in North America, particularly Passeriformes and Anseriformes. Haemoproteus and Leucocytozoon occur most frequently, although various species of Plasmodium and, occasionally, other less known forms are recognized. Prevalence of these parasites in wild birds is related to season of year and age of host. Highest incidence occurs in spring and summer. Relapse of Haemoproteus and Leucocytozoon is common in the spring. Blood smears of adult wood ducks, on the Atlantic flyway, in April and May show a high prevalence of Haemoproteus, while smears at other times are usually negative. Although the author (Herman, 1938) demonstrated that young redwings in Massachusetts primarily acquired Plasmodium infections after leaving the nest, in many cases infection is acquired by the nestling. Nestling magpies in northeastern California acquire a high incidence of infection with several parasites. The hypothesis, expressed by Manwell and Herman (1935), that a higher prevalence of infection can be expected in more southerly ranging species, is subject to question. Smears taken during the winter demonstrate higher parasite prevalence in birds at the southern limits of their range, such as juncos and white-throated sparrows, than do smears of other species with more southern range. Little is known of significance of these parasites to survival of the host, although O'Roke ( 1934) reported a high loss for ducklings from Leucocytozoon and there have been occasional reports of fatality in other species.