The life-cycle of many strigeoid trematodes contains encysted metacercariae as disseminating and infective stages. The cyst wall may vary in form from a thin bag-like structure loosely covering the metacercaria (as in Posthodiplostonmm) to a thick layer, closely investing the parasite and forming a cyst of fixed outline. The cyst wall enclosing the metacercaria of Cyathocotyle is of this thick, rigid type. Extensive histochemical studies have been carried out on the cyst wall of Posthodiplostonmm (Hunter and Hunter, 1940; Bogitsh, 19G2), but the characteristics of the other types of strigeid cyst wall are as yet undefined.