Observations were made on the structure of the oncosphere of Taenia crassiceps. The oncosphere contained (1) a pair of unicellular glands which stained with thionine and toluidine blue; (2) pyroninophilic cells, containing large vesicular nuclei when stained with haematoxylin; (3) smaller non-pyroninophilic cells, with less distinct nuclei; (4) poorly staining fibrous elements, presumably muscular; and (5) three pairs of hooks.