Fear is an integral part of predator-prey interactions with cascading effects on the structure and function of ecosystems. Fear of parasitism holds a similar ecological potential but our understanding of the underlying mechanisms in host-parasite interactions is limited by lack of empirical examples. Here, we experimentally test if blue mussels Mytilus edulis respond behaviourally to the mere presence of infective transmission stages of the trematode Himasthla elongata by ceasing filtration activity, thereby avoiding infection. Our results show that blue mussels reduced clearance rates by more than 30% in presence of parasites. The reduced filtration activity resulted in lower infection rates in experimental mussels. The identified parasite-specific avoidance behaviour can be expected to play a significant role in regulating the ecosystem engineering function of blue mussels in coastal habitats.