The prevalence of hematozoa (diagnosed from blood films) in a sample of 4200 anatids representing 14 species collected in New Brunswick, Nova Scotia and Prince Edward Island during the period 1969-1973 is given. Thirty percent of the birds harboured hematozoa; the commonest blood parasite was Haemoproteus (Parahaemoproteus) nettionis (in 18% of the sample), followed by Leucocytozoon simondi (in 14% of the sample). Four species of Plasmodium occurred in 5% of the sample—Plasmodium circumflexum was the most frequently encountered of the four. Prevalence of infection varied markedly from year to year and locality to locality. Ducks from Northwestern New Brunswick-Bathurst area were the most heavily infected while those from Prince Edward Island had a low hematozoan prevalence. Black ducks (Anas rubripes) were the most heavily infected of the 14 species of waterfowl sampled and possibly hematozoa may act as a limiting factor on populations of this duck. Bluewing teal (Anas discors) showed the lowest prevalence (7.1%) of the 14 species studied and the prevalence was in marked contrast to that of the greenwing teal (Anas carolinensis) (39.5%).