The Attwater's prairie chicken (APC; Tympanuchus cupido attwateri Bendire, 1894) has been a federally listed endangered species since 1967. Several captive propagation programs consisting of small populations are being used to keep this species from extinction. Fecal samples were collected from APCs in April 2007 and again in August 2008 from 2 separate captive propagation facilities in Texas after clinical signs of coccidiosis were observed. One Eimeria species was observed (Eimeria attwateri), which we describe as a putative new species. Sporulated oocysts are ellipsoidal, 30.0 × 18.4 (27.4–31.3 × 16.0–22.4) µm. Oocysts have a smooth wall 0.7 µm thick and lack both a micropyle and oocyst residuum, but 1 ellipsoidal polar granule is present, 2.3 × 1.9 (2.1–2.4 × 1.7–2.0) µm. Sporocysts have a nipple-like Stieda body with a rounded opposite end and are 14.0 × 7.1 (10.2–16.8 × 6.0–9.2) µm. The sporocysts contain a sporocyst residuum usually consisting of 2–4 dispersed globules, and each sporozoite contains 2 large posterior spheroid refractile bodies 3.4 µm wide. Nucleotide sequence amplified from the 18S rDNA does not match any DNA sequence information for publicly available Eimeria species, and phylogenetic reconstructions place this species with other eimerians from Galliformes. The discovery of a potentially pathogenic species of Eimeria in captive APCs is of great importance, and managers should be aware of the potential devastating effect(s) this parasite could have on the APC conservation programs.