Caryospora cheloniae sp. n. is described from mariculture-reared green sea turtles (Chelonia m. mydas). The sporulated oocyst has a thin, transparent, single-layered wall which often ruptures, leaving a naked sporulated sporocyst. Oocysts measured 33.8 to 40.1 μm by 11.0 to 14.6 μm (mean 37.4 by 12.8 μm). Greatest concentrations of developmental stages of C. cheloniae were found in the hindgut. Transverse binary fission was observed in dividing tissue stages. Pathologic alterations were most pronounced in the posterior third of the intestines (hindgut). The hindgut lumen was greatly dilated and filled with blood, oocysts and tissue debris. The hindgut wall was thinner than normal and the mucosal folds had sloughed into the intestinal lumen. Free blood escaped from the blood vessels of the tunica propria into the intestinal lumen. Epithelial hyperplasia was pronounced at the margins of denuded mucosal areas. Numerous inflammatory cells infiltrated the infected mucosal surface.