Faecal samples from 23 adult and 20 young captive Yemen chameleons, Chamaeleo calyptratus Duméril et Duméril, 1851, were examined for coccidian parasites. Two of the adult (8.7 %) and 16 (80.0 %) of the young chameleons were found to be passing oocysts of the Isospora species. Sporulated oocysts of Isospora jaracimrmani sp. n. are ellipsoidal or pyriform, 38.4 x 25.6 (35.2-42.8 x 23.8-27.0) pm, with smooth bilayered and colourless oocyst wall. A micropyle, oocyst residuum and polar granule are absent. Sporocysts are ellipsoidal to ovoid, 15.9 x 11.2 (14.8-17.0 x 10.4-12.0) pm, with dome-like Stieda and homogenous spherical substieda bodies. The posterior end of the sporocyst is draw out. Sporocyst residuum is present, consisting either of a compact mass or of scattered granules. Sporozoites, with faint transverse striations anteriorly, are vermiform, 13.1 x 3.4 (12.4-16.5 x 2.8-4.0) pm. Most oocysts are to sporulate when excreted; sporulation was completed within 12 to 24 h at 25 ± 2"C. Endogenous stages develop inside the nuclei of enterocytes in the small intestine. Prepatent period in experimentally infected young chameleon was 7 days. Comparison with other species of the genus Isospora found in chameleons indicates that it is a new species.