In vitro experiments showed that the oyster mushroom Pleurotus pulmonarius exerts a strong immobilising effect on preinfective larvae of the animal parasitic nematodes Ostertagia ostertagi, Cooperia oncophora, Oesophagostomum quadrispinulatum and Cyathostoma species. Infective stages of the same species seemed less vulnerable to fungal attack, possibly because they are protected by their outer cuticle.