Although host parasite interactions can affect the ecology and distribution of species, virtually nothing is known about range-wide variation in haematozoan levels in North American birds. We investigated haematozoan prevalence and parasitemia in 112 adult male cerulean warblers (Dendroica cerulea (Wilson, 1810)) in six populations throughout this species' breeding range. Thirty-six birds were infected with Haemoproteus paruli at low parasitemia (mean ± 1 SE, 6.18 ± 1.27 parasites/2000 erythrocytes). Prevalence did not differ between age classes, and there was no significant relationship between parasitism and body condition. However, prevalence and parasitemia were significantly higher in southern populations and further study is required to elucidate factors underlying regional differences in cerulean warbler haematozoan levels.