In order to determine whether the miracidia of Euparyphium albuferensis and Echinostoma friedi are sensitive to their host snail (HS) and capable of discriminating between HS and non-host snails (NHS), or whether these NHS can interfere and thus reduce the infection rates (decoy effect), a total of three experiments were conducted with HS, NHS and snail-conditioned water (SCW). Gyraulus chinensis is the HS for E. albuferensis miracidia, while Physa acuta, Radix peregra and Lymnaea fuscus are considered NHS. For E. friedi miracidia R. peregra, G. chinensis and L. fuscus are the HS, while P. acuta is the NHS. The NHS R. peregra produces the greatest decoy effect on Euparyphium albuferensis miracidia, while R. peregra, as the HS of Echinostoma friedi miracidia, is always subject to a NHS decoy effect. However, an increase in E. friedi miracidia infectivity is observed in its HS G. chinensis, in the presence of SCW of P. acuta. These experimental results might explain the low prevalence of snails infected with E. albuferensis miracidia in their natural habitat.