Large populations of the new species of Entodinium were found in 27 out of 120 stomachs (from the paunch to reticulum) of Bos taurus from various regions of Yugoslavia. The stomach content was fixed in 4 per cent. formaldehyde. Unstained glycerine mounts and material stained in iron haematoxylin and Mallory's phosphor-molybden haematoxylin were used. Twenty individuals from each population, i.e. 540 individuals in all, were measured. The length was found to vary from 26·25 to 38·5μ, and the width from 24·5 to 31·5μ. The relation between the length and the width was 1·16, which indicates the average degree of rotundity. This rotundity, especially noticeable in individuals in which the adoral membranelle zone is retracted, may sometimes be less pronounced at the anterior end. Entodinium bovis is considerably flattened, so that its thickness does not exceed 6–10μ. The pharynx is curved dorsally. The cuticle is longitudinally striated. The ectoplasm is more developed in the anal region than laterally. The macronucleus is elongated, 10·5–21 × 3·5–5×25μ, and is thicker at its anterior end. The micronucleus is round and lies ventral to the macronucleus at a point about the middle of its length. The contractile vacuole lies ventral to the macronucleus, near to its anterior end. The anus is usually situated in the middle of the posterior end of the ciliate, though sometimes it lies slightly ventral.