1. Endamœba citelli sp. nov. is a commensal in the cœcum of the striped ground squirrel, Citellus tridecemlineatus.2. The nucleus of the free form is typical of the genus Endamœba.3. The cyst is characterized by a nuclear structure somewhat different from that known for other amœbæ, and an unusually thick cyst wall with a yellowish refraction.4. The cytoplasm of this amœba may contain a cytridine parasite, Sphærita endamœbæ sp. nov.5. The developmental cycle of this cytozoic parasite was followed from the free bacterium-like infective stage to the spore liberated from the sporangium inside the cytoplasm of the amœba.6. Sphærita endamœbæ, like other members of the genus, produces degenerative changes in the protoplasm of its host, particularly in the nucleus.