A new caryosporan (Apicomplexa: Eimeriidae) from flat-headed snake, Tantilla gracilis (Reptilia: Ophidia: Colubridae), from McCurtain County, Oklahoma, USA.

The flat-headed snake, Tantilla gracilis Baird and Girard is a small reptile belonging to the family Colubridae. Feces from a single T. gracilis from McCurtain County, Oklahoma, USA, was collected and examined for coccidia; it was found to be passing a new species of Caryospora. Oöcysts of Caryospora sargentae n. sp. are spheroidal to typically subspheroidal with a moderately-pitted bi-layered wall, measure (L × W) 20.7 × 19.2 µm, and have a length/width (L/W) ratio of 1.1; a micropyle and oöcyst residuum were absent but one to several small, irregular to spheroidal masses of non-refractile debris within the oocyst was present as well as a polar granule attached to the inner oöcyst wall. Sporocysts are ovoidal and measure 15.0 × 11.0 µm, L/W 1.4; a nipple-like Stieda body is present as well as a distinct rounded sub-Stieda body. The sporocyst residuum is composed of a large, dense, irregular mass of various sized granules located between and often obscuring the sporozoites. This is the third coccidian described from the flat-headed snake. In addition, we include some data on the endogenous development of the coccidian.