Forty-nine olive-backed pocket mice, Perognathus fasciatus were collected during 2011 and 2012 from 4 sites in Wyoming and examined for coccidian parasites. Fifteen (31%) were found to be passing oocysts of a new species of Eimeria Schneider, 1875. Sporulated oocysts of Eimeria fasciata n. sp. are ellipsoidal to ovoidal, 23.3 × 20.7 (19–27 × 17–25) µm, with a shape index of 1.1; they typically contain a single, smooth, bubble-like oocyst residuum. Oocysts possess 1–2 polar granules, lack a micropyle, and are bilayered with a thickness of 1.3 µm. Sporocysts are ovoidal, 10.0 × 8.2 (8–12 × 7–10) µm, with a shape index of 1.2; they contain a sporocyst residuum that appears similar to a cluster of 1–8 grapes. The Stieda body is small, appearing flattened to knobby, and there are no subStieda or paraStieda bodies. This new eimerian represents the only coccidian, to date, reported from P. fasciatus, as well as the only species from any heteromyid rodent in Wyoming.