A feature of fascioliasis in mice and rats is incomplete and generally poor resistance to reinfection. The possibility exists that population heterogeneity amongst Fasciola hepatica parasites (in characteristics such as antigenic variability and infectivity) contributes to the incomplete resistance expressed by already infected rodents to challenge infection. Using different exposure regimes, mice and rats were dosed with infective metacercariae of different single snail-derived clones and challenged with the same or different clonal parasites. The results clearly demonstrate that no better resistance to reinfection is seen with parasites of the homologous clone than with heterologous clone challenge. Thus the poor resistance to reinfection seen in fascioliasis cannot be ascribed readily to antigenic or infectivity differences between clonal metacercariae.