Blood parasites of blue grouse (Dendragapus obscurus) were sampled and the factors responsible for variation in prevalence of blood parasites, and patterns of association among parasite species, were investigated. Five genera of haematozoa were surveyed including four protozoans (Haemoproteus, Leucocytozoon, Plasmodium, and Trypanosoma) and a nematode (Splendidofilaria). Prevalence of blood parasites varied significantly between years; sexes differed in number of parasite species in one of two years. Both positive and negative 'overall' associations among all parasites were found when variance-ratio tests were used indicating that blood parasites often were not independent of one another. In general, Leucocytozoon and Trypanosoma often co-occurred; the strongest associations between these two parasites appeared in samples of hosts most heavily infected by other parasites. Negative associations between parasite species always involved Haemoproteus. Associations between pairs of species did not account wholly for overall patterns of associations within the parasite assemblage. Studies of associations within blood parasite assemblages are important for understanding the ecology of haemotropic infections and for clucidating the need for multi-parasite models of parasite-host interactions.