During a survey of the coccidian parasites of reptiles, caryosporan oocysts were found in the faeces of wild and captive European viperid snakes Vipera berus (L.) and V. ammodytes (L.). Thirty two of 37 examined V. herus (86%) and 9 of 17 examined V. ammodytes (53%) specimens were found to be passing caryosporan oocysts. Morphological characters of all caryosporan isolates were identical and fitted well with the description of Caryospora simplex Léger, 1904. Experimental inoculation of severe combined immunodeficient (SC1D) mice with seven isolates of C. simplex from V. berus or V. ammodytes confirmed the heteroxenous life cycle pattern, for the first time for isolates of evidently European origin. Caryosporan developmental stages were observed in the connective tissues of the nose, cheeks, ear and scrotum in all inoculated SCID mice. V. berus and V. ammodytes represent new hosts for C. simplex. The present paper represents the first widely based report on coccidian parasites of the genus Caryospora Léger in European viperids. Our findings indicate a wide distribution of C. simplex throughout the range of distribution of snakes of the genus Vipera.