Early development of the coccidium Sarcocystis muriviperae Matuschka, Heydom, Mehlhom, Abd-Al-Aal, Diesing et Bichler, 1987 is described from experimentally infected white mice fed sporocysts from naturally infected Vipera palaestinae and Coluber jugularis. Although the course of infection was similar, mice infected with the sporocysts from the first host survived an inoculum of up to 200,000 sporocysts, while others infected with the second, succumbed to inocula exceeding 40,000 sporocysts in 7-10 days post infection (p,i,). Histological and ultrastructural studies revealed merogony in the hepatocytes during days 7-10 p.i. and onset of sarcocyst development by days 19-21 p.i. The livers of infected mice are grossly enlarged and of a mottled whitish colour due to severe neutrophil inflammatory infiltration, apparently stimulated by host cell residues or from defunct disaggregating meronts at the end of the merogony cycle. Early sarcocysts undergo a further division by endopolygeny before proceeding to division by endodyogeny.