Natural infestation of Lymnaea truncatula by Fasciola hepatica was studied at various sites in the traditional oasis of Tozeur 8 times between September 1997 and August 1998. Infestation of snails was documented in five of the eight sites with a mean level of 26.1 p. 100. Level of infestation varied depending on the site. It was highest in secondary irrigation canals (32.8 p. 100), seguias, (34.8 p. 100), and secondary drainage canals (34.8 p. 100). Lower levels were observed in the main drainage canal, (15.2 p. 100). Two infestation periods were noted at two sites in relation with spreading of manure by farmers in autumn and spring. Perennial infestation due to the permanent presence of animal hosts was noted at one. Infestation of drainage canals is enhanced by incoming parasite eggs from upstream irrigation canals. Infestation was age dependent reaching up to 100 p. 100 in older snails of 5.5 mm. Environmental factors such as climatic conditions and human intervention are important to sustaining snail infestation in the oasis of Tozeur.