In this paper we stress the important role that immature parasites can play in the interaction with their hosts and the importance of looking at juvenile hosts when studying parasite effects. These results are discussed in relation to the evolution of host-parasite associations and to Hamilton & Zuk's (1982) hypothesis on the evolution of secondary sexual characters. We exposed juvenile European minnows Phoxinus phoxinus from two lake populations to sympatric and allopatric Diplostomum phoxini in a reciprocal cross-infection experiment. Parasites from both localities were more infective to the sympatric hosts than to the allopatric ones. During the first 4 weeks after infection, when the parasites grow within the host, minnows from both sympatric host-parasite combinations grew less quickly than minnows infected with allopatric parasites