Protozoan parasites of the genus Klossiella have been reported in the mouse (Klossiella muris) by Smith and Johnson (1902) and in the guinea pig (Klossiella cobayae) by Seidelin (1914). Wenyon (1926) stated that a Klossiella, species undetermined, has been found in the kidney of the jerboa, a South African rodent. The purpose of this article is to describe Klossiella equi, n. sp., in the kidney of an American Jack, Equus asinus. The host of the parasite was a jackass, 2'2 years old, that died from spinal injuries incurred while he was being roped. Post-mortem examination revealed no gross evidence of any disease condition affecting the thoracic or abdominal viscera. Routine histologic examination was made of the brain, liver, and kidneys. Following the discovery of the sporogonic cycle of a protozoan parasite in the kidneys, a search was made for schizogonic forms in the capillary endothelial cells of the liver and brain, but none were found. The parasite was found within epithelial cells lining the thick limbs of Henle's loops situated in the medullary rays of the kidneys. There were 4 morphological variations of the parasite apparently representing 4 stages in development. Three of the variations are designated by descriptive terms as follows: (1) uninuclear form, (2) multinuclear form, and (3) budding form. The fourth variation consisted of developing spores within the parasitized epithelial cell. Description of the developing oocyst: (1) The uninuclear form (Fig. 1) was a round or slightly oval mass of reticulated protoplasm averaging 11 microns in diameter in formalin-fixed paraffin-embedded tissue. There was a small nucleus at the periphery, measuring approximately 3 microns in diameter and containing 1 or more small basophilic particles. Each parasite was located within a vacuole in the cytoplasm of its enlarged renal epithelial host-cell. The vacuole evidently was an artifact produced by shrinkage from fixing, dehydrating, and embedding the tissue for histologic section. (2) The multinuclear form was slightly larger than the uninuclear form, ranging from 18 to 22 microns in diameter. It consisted of a round or oval mass of reticulated protoplasm with multiple nuclei arranged along the border (Fig. 2). The nuclei were small, not very distinct and consisted of two small basophilic granules surrounded by a vaguely defined small area of condensed protoplasm. The epithelial cell containing the parasite was greatly enlarged and distorted, but not to such an extent that its identity was obscured. The multinuclear form differed from the uninuclear form in that it was slightly larger and that proliferation of the nucleus had occurred. (3) The budding form varied from 30 to 40 microns in greatest diameter; the smaller individuals were round and the larger ones ovoid. It was enclosed in a membrane apparently com-