AbstractThe formation of capture organs for nematodes in A. oligospora. and its dependence on substrate composition and additions of animal origin was studied in laboratory experiments. Variations in only the substrate composition did not give rise to capture organ formation. If nematodes were present, however, the medium composition was of great importance for the number of traps formed. Capture organs were not only formed in the presence of nematodes but also by addition of other taxonomically diverse animals such as earthworms, fly‐larvae and spring‐tails. The initialing factor is, for full effect, bound to the living state and not confined to nematodes. For biological control of nematodes with the aid of nematode‐trapping fungi, therefore, the effect of the other microfauna of the soil should also be taken into consider