A single flathead snake, Tantilla gracilis, collected in early October 2010 from Choctaw County, Oklahoma, was found to harbor an undescribed species of Caryospora. Oocysts of Caryospora choctawensis n. sp. were spherical to subspherical, 15.8 × 15.0 (14–18 × 14–16) µm, with a thick, bilayered wall and a shape index (length∶width) of 1.1. A micropyle and an oocyst residuum were absent, but prominent Stieda and bubble-like sub-Stieda bodies were present as well as a bilobed polar granule near the oocyst wall. Sporocysts were ovoidal, 10.8 × 9.0 (10–12 × 8–9) µm, with a shape index of 1.2. The sporocyst residuum was spherical and composed of a cluster of granules often membrane-bound. This is the second time a caryosporan species has been reported from T. gracilis but the first coccidian ever described from a reptilian host in Oklahoma. Additional T. gracilis from Arkansas (n  =  6), Oklahoma (n  =  1), and Texas (n  =  7) were examined, and a single specimen from Newton County, Arkansas harbored Caryospora gracilis Upton, McAllister, Trauth, and Bibb, 1992, previously reported from T. gracilis collected in Arkansas and Texas.