Three types of gland cells have been characterised histochemically in the cercariae of Stictodora lari. The central gland cells, which appear early in development, secrete their contents to form a body film soon after the cercariae leave the redia; the penetration gland cells appear to be concerned solely with penetration of the fish host; the peripheral gland cells are of unknown function. None of these gland cells appear to contribute material to the cyst wall of the metacercaria. Initial cyst walls formed around metacercariae in vivo and in vitro are considered identical and derived from a thin, vacuolated layer which surrounds the body of the cercaria. About 35 days after encystment a cellular host capsule is formed around the initial cyst wall. This is followed by the deposition of a layer of protein around the metacercaria, derived from granules synthesized in the cells of the bladder wall. The initial cyst wall could not be detected after the formation of the host capsule and deposition of the protein layer; thus the completed cyst wall consists of at least 2 layers—an outer cellular capsule of host origin and an inner protein layer of parasite origin.