The mudsnail Hydrobia ulvae, primary host to a number of digeneans, was studied to check whether parasite effects on individual hosts could have repercussion on population size structure and dynamics. Three different mudsnail populations were monitored monthly in Arcachon Bay, south-west France. Arguin is a moderately sheltered oceanic sandflat and a bird reserve, La Canelette is a sheltered sandflat near a harbour and Lette Douce is a sheltered salt marsh. Parasite prevalence differed among the three stations, with no obvious seasonal pattern: 0–16% at Arguin, 0–6% at La Canelette and 1–5% at Lette Douce. Between four and five digenean species were identified, with Haploporidae dominating at Arguin and Notocotylidae dominating at Lette Douce. Prevalence increased with snail shell height. Large snails from Arguin were heavily parasitized, but suddenly disappeared between December 1998 and January 1999. The snail size distribution was consequently modified. To test whether digenean-induced mortality could be involved, a 50-d laboratory experiment was performed. Mudsnails hosting digeneans with rediae (as Haploporidae or Heterophyidae) displayed significantly higher mortality rates than mudsnails hosting sporocysts (as Microphallidae) or healthy mudsnails. The present study demonstrates that digenean parasites induce population structure changes and shorten lifespan of mudsnails, through increasing size-dependent mortality.