With fewer than 50 birds remaining in the wild, Attwater's Prairie-Chicken (Tympanuchus cupido attwateri) is critically endangered. Individuals of this species on the Attwater Prairie Chicken National Wildlife Refuge, Colorado Co., TX, have been attacked in successive winters, 2005-2006, by the blood-feeding black fly Cnephia ornithophilia. Attwater's Prairie-Chicken is a previously unreported host for Cnephia ornithophilia. Molecular screening indicated that about 15% of 13 blood-fed flies sampled from captured Attwater's Prairie-Chickens carried a parasite of the genus Leucocytozoon that can cause a debilitating avian malaria-like disease. If blood feeding or transmission of the disease agent becomes a threat to the birds, particularly in years of lean food supply or harsh weather, management of Cnephia ornithophilia should be considered.