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.