Chilomastix megamorpha n. sp., is described from the caecum of the Indian guinea-pig. In its dimensions, the new organism is the largest among the mammalian species. The length ranges (in microns) from 24.0–42.6, average 31.6; the breadth ranges from 10.0–20.5, average 15.3. The normal shape of the body is elongated-oval, with a pointed anterior end and a spike at the posterior end. The nucleus is chestnut in shape and contains endosomal substance which is constant in position. Four blepharoplasts are present: the first three in a row at the anterior margin of the body, the fourth close to the anterior tip of the nucleus. Three anterior flagella arise from the first three blepharoplasts. The cytostomal flagellum and the cytostomal fibrils arise from the fourth blepharoplast.