Two juvenile snowy owls (Nyctea scandiaca) died after short episodes of lethargy and anorexia. Postmortem examination of the first was inconclusive but clinical and laboratory examinations on the second owlet revealed a high level of erythrocyte parasitaemia by Haemoproteus noctuae. After a period of intensive nursing and chemotherapy during which it improved clinically and the red cell parasitism decreased, the bird's condition relapsed and it died. Postmortem examination revealed H noctuae infection in addition to heavy red blood cell parasitism by Leucocytozoon ziemanni. Although neither H noctuae nor L ziemanni are generally considered to be pathogenic, their combined presence in the young owl apparently resulted in its death.