A sample of mud snails Hydrobia ulvae (Prosobranchia) from an intertidal po
pulation revealed that the shells of trematode-infected specimens were espe
cially likely to be fouled with epibionts. Experimentally trematode-infecte
d Biomphalaria glabrata (Pulmonata) appeared to be especially prone to deve
lop epigrowth in comparison with uninfected conspecifics as well. These fin
dings suggest an interaction between trematode infections and epibiosis in
aquatic gastropods. The two most likely explanations for this are (1) that
trematode infections weakens the snails' natural defences against epibionts
, or (2) that the defences against epibionts also are effective against inv
ading trematodes, causing snail specimens with a particularly good fouling
defence to be less likely to become infected.