Ultrastructural studies on Crepidostomum farionis have been undertaken in order to elucidate the development of the redia, cercaria, metacercaria and juvenile stages in the intermediate and definitive hosts. Particular attention has been paid to the tegument, gastrodermis, excretory vesicle and glandular systems. Concurrent investigations into the bionomics and behaviour of the larval stages have also been carried out. Rediae have been shown to have a birth pore situated at the posterior end of the body and, additionally paracrystalline structures which may be viruses have been detected in them. The tegument of the cercaria appears to arise by a process of delamination from the primitive epithelium covering the germ ball. Secretory bodies passed into the cercarial tegument, while the cercaria is still inside the redia, may play a part in cyst formation and secretory bodies in the ventral sucker tegument may be important in adhesion to Gammarus. After excystment these secretory bodies are no longer evident in the tegument and new cytons producing different secretory bodies make contact with. the tegument. Penetration glands open into the stylet sac of cercariae; similar glands open at the anterior end of metacercariae and juveniles but there are indications that their secretory activity diminishes as growth proceeds. They may be involved in extracorporeal digestion in juveniles before completion of development of the gastrodermis. Two forwardly directed pigment cup ocelli have been described in the cercaria and presumed sensilla$ of various kinds have been studied in cercariae, metacercariatand juveniles. Anteriorly situated lappets of the metacercariae and juveniles are almost entirely muscular. Secretory bodies produced by the cercarial bladder epithelium and absent after encystment may play a part in cyst formation. Concretions which accumulate in the bladder of the metacercaria are shed after excystment, lipid excretion begins and a thick glycocalyx appears on the tegument lining the excretory canal. Cercariae, shown to emerge during the hours of daylight, are strongly photonegative and geopositive. Metacercariae excyst readily in 1% trypsin solutions at alkaline pH.