A nonsuppurative meningoencephalitis, previously presumed to be toxoplasmosis, was found in 53 capercaillies (Tetrao urogallus L.) examined at necropsy at the National Veterinary Institute, Uppsala, Sweden, between 1966 and 1985. Pronounced meningitis and encephalitis with perivascular cuffs of mononuclear inflammatory cells as well as focal gliosis were prominent histopathologic findings. Protozoa were frequently associated with these lesions. Ultrastructurally, the protozoa appeared to divide by endopolygeny, and merozoites had no rhoptries. Organisms from all 12 birds subjected to Sarcocystis cruzi immunohistochemical staining reacted positively but did not react to Toxoplasma gondii antiserum. The agent was, therefore, assigned to the family Sarcocystidae and was probably more closely related to species of the genus Sarcocystis than to T. gondii.