The fine structure of various parts of the hydatid cysts and the enclosed protoscolex is described. The contents of the brood capsule are PAS-positive following extraction treatment with amylase. This material is made up of fine irregularly arranged filaments, and remains as a coating over the posterior aspect of the protoscolex after its removal from the brood capsule. It has completely disappeared after 42 days in vitro. The posterior aspect of the protoscolex is covered with knoblike projections which are probably undeveloped tegumental projections. After 42 days culture in vitro this area has fully developed tegumental projections. Multinucleation of tegumental cells is common in the protoscolices cultured for 42 days in vitro. The various organ systems are described and compared with similar structures for tapeworms and other invertebrates.