Oocyst morphology and endogenous developmental stages are described for Choleoeimeria salaselensis sp. n. from the gall bladder of 10 horned vipers, Cerastes gasperettii Leviton and Anderson, in Saudi Arabia. Sporulated oocysts are ellipsoidal, 23 x 15 (22-25 x 14-17) microm, length/width ratio (L/W) 1.5 (1.4-1.6), each with 4 sporocysts (Eimeria-like), but lack a micropyle, polar granules and oocysts residuum. Sporocysts are ellipsoidal, 8 x 5 (7-9 x 5-) microm, L/W 1.5 (1.4-1.6), and Stieda, substieda and parasubstieda bodies are all absent, but a longitudinal suture, which divided the sporocysts into 2 plates, is present. Endogenous development is confined to epithelial cells in the bile duct and gall bladder; mature meronts were 11 x 7 microm, each with 10-16 merozoites, microgamonts were -12 microm wide, and macrogamonts were -16 microm wide with a prominent nucleus and wall-forming bodies. Given these two diagnostic features, sporocysts with a suture and composed of two plates and endogenous development limited to the biliary epithelium, we believe this coccidium is best classified as a member of Choleoeimeria Paperna et Landsberg, 1989. There are 5 known Eimeria species from vipers that have sporocysts somewhat similar in size to those of our new form, but all of them have much larger oocysts and larger sporocysts, some of which differ significantly in shape; there are not yet any Choleoeimeria species known from the Viperidae.

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