The life cycle of a senegalese strain of Fasciola gigantica was carried out in France from adults recovered in the liver of naturally infected cattle (Bos taurus). Laboratory-reared Lymnaea natalensis experimentally infected shed numerous cercariae. Metacercariae were used for successful infection of sheep. In the miracidium, variations are observed in the disposition of the medio-ventral epithelial cell of the second tier. The distribution of the superficial argentophilic structures is similar to that of the miracidium of F. hepatica. The chaetotaxy of the cercaria is very closely related to that of the cercaria of a strain of F. gigantica from Madagascar shed by Lymnaea hovarum but it differs clearly from that of the cercaria of F. hepatica. The encystment of a great number of cercariae (25 %) on the surface of water suggests that this is an important factor in the extension of the foci of contamination, and therefore increases greatly the risks of infection not only for animal but also for man. It is possible that human fascioliasis caused by F. gigantica is a disease, the importance of which is under-estimated.